FAIR VALUE OF FINANCIAL INSTRUMENTS (Financial Instruments Measured at Fair Value) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	$ 1,825,438	$ 1,719,607
Trading and Equity Securities	0	15,256
U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	149,938	69,491
U.S. government agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	64,518	100,202
Foreign bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	29,602	5,117
Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	83,502	63,651
Corporate bonds | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	25,336	30,124
Corporate bonds | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	43,257	54,015
Corporate bonds | Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	50,567	49,466
Corporate bonds | Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	7,576	9,480
Corporate bonds | Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	41,636	27,670
Corporate bonds | Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	101,031	100,253
Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	20,577	6,356
Collateralized mortgage obligations | Government national mortgage association
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	86,152	80,356
Collateralized mortgage obligations | Federal home loan mortgage corporation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	152,843	124,502
Collateralized mortgage obligations | Federal national mortgage association
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	83,282	71,845
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	926	750
Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	1,825,443	1,719,607
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	206,685	15,256
Short-Term Investments	175	175
Money Market Accounts	24,790	9,334
Corporate-Owned Life Insurance	8,557	6,777
Total Assets Measured at Fair Value	2,065,650	2,050,352
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	206,090	4,087
Short-Term Investments	175	175
Money Market Accounts	24,790	9,334
Corporate-Owned Life Insurance	8,557	0
Total Assets Measured at Fair Value	239,612	312,204
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	0	11,169
Short-Term Investments	0	0
Money Market Accounts	0	0
Corporate-Owned Life Insurance		6,777
Total Assets Measured at Fair Value	1,824,267	1,736,553
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	595	0
Short-Term Investments	0	0
Money Market Accounts	0	0
Corporate-Owned Life Insurance	0	0
Total Assets Measured at Fair Value	1,771	1,595
Recurring | U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	149,938	69,491
Recurring | U.S. Treasury | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | U.S. Treasury | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	149,938	69,491
Recurring | U.S. Treasury | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | U.S. government agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	64,518	100,202
Recurring | U.S. government agency | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | U.S. government agency | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	64,518	100,202
Recurring | U.S. government agency | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | States, municipalities and political subdivisions | General obligations | Midwest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	81,980	88,594
Recurring | States, municipalities and political subdivisions | General obligations | Northeast
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	30,450	31,270
Recurring | States, municipalities and political subdivisions | General obligations | South
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	109,970	115,203
Recurring | States, municipalities and political subdivisions | General obligations | West
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	110,021	110,317
Recurring | States, municipalities and political subdivisions | Special revenue | Midwest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	125,098	139,892
Recurring | States, municipalities and political subdivisions | Special revenue | Northeast
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	61,076	61,543
Recurring | States, municipalities and political subdivisions | Special revenue | South
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	226,706	234,666
Recurring | States, municipalities and political subdivisions | Special revenue | West
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	139,399	144,844
Recurring | States, municipalities and political subdivisions | Level 1 | General obligations | Midwest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | States, municipalities and political subdivisions | Level 1 | General obligations | Northeast
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | States, municipalities and political subdivisions | Level 1 | General obligations | South
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | States, municipalities and political subdivisions | Level 1 | General obligations | West
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | States, municipalities and political subdivisions | Level 1 | Special revenue | Midwest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | States, municipalities and political subdivisions | Level 1 | Special revenue | Northeast
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | States, municipalities and political subdivisions | Level 1 | Special revenue | South
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | States, municipalities and political subdivisions | Level 1 | Special revenue | West
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | States, municipalities and political subdivisions | Level 2 | General obligations | Midwest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	81,980	88,594
Recurring | States, municipalities and political subdivisions | Level 2 | General obligations | Northeast
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	30,450	31,270
Recurring | States, municipalities and political subdivisions | Level 2 | General obligations | South
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	109,970	115,203
Recurring | States, municipalities and political subdivisions | Level 2 | General obligations | West
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	110,021	110,317
Recurring | States, municipalities and political subdivisions | Level 2 | Special revenue | Midwest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	125,098	139,892
Recurring | States, municipalities and political subdivisions | Level 2 | Special revenue | Northeast
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	61,076	61,543
Recurring | States, municipalities and political subdivisions | Level 2 | Special revenue | South
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	226,706	234,666
Recurring | States, municipalities and political subdivisions | Level 2 | Special revenue | West
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	139,399	144,844
Recurring | States, municipalities and political subdivisions | Level 3 | General obligations | Midwest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | States, municipalities and political subdivisions | Level 3 | General obligations | Northeast
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | States, municipalities and political subdivisions | Level 3 | General obligations | South
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | States, municipalities and political subdivisions | Level 3 | General obligations | West
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | States, municipalities and political subdivisions | Level 3 | Special revenue | Midwest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | States, municipalities and political subdivisions | Level 3 | Special revenue | Northeast
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | States, municipalities and political subdivisions | Level 3 | Special revenue | South
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | States, municipalities and political subdivisions | Level 3 | Special revenue | West
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Foreign bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	29,602	5,117
Recurring | Foreign bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Foreign bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	29,602	5,117
Recurring | Foreign bonds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	83,502
Recurring | Public utilities | Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)		63,651
Recurring | Public utilities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0
Recurring | Public utilities | Level 1 | Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)		0
Recurring | Public utilities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	83,502
Recurring | Public utilities | Level 2 | Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)		63,651
Recurring | Public utilities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0
Recurring | Public utilities | Level 3 | Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)		0
Recurring | Corporate bonds | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	25,336	30,124
Recurring | Corporate bonds | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	43,257	54,015
Recurring | Corporate bonds | Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	50,567	49,466
Trading and Equity Securities		2,276
Recurring | Corporate bonds | Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	7,576	9,480
Trading and Equity Securities		4,701
Recurring | Corporate bonds | Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	41,636	27,670
Trading and Equity Securities		1,732
Recurring | Corporate bonds | Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	101,031	100,253
Trading and Equity Securities		2,460
Recurring | Corporate bonds | Level 1 | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Corporate bonds | Level 1 | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Corporate bonds | Level 1 | Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Trading and Equity Securities		0
Recurring | Corporate bonds | Level 1 | Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Trading and Equity Securities		0
Recurring | Corporate bonds | Level 1 | Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Trading and Equity Securities		0
Recurring | Corporate bonds | Level 1 | Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Trading and Equity Securities		0
Recurring | Corporate bonds | Level 2 | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	25,336	30,124
Recurring | Corporate bonds | Level 2 | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	43,257	54,015
Recurring | Corporate bonds | Level 2 | Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	50,567	49,466
Trading and Equity Securities		2,276
Recurring | Corporate bonds | Level 2 | Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	7,576	9,480
Trading and Equity Securities		4,701
Recurring | Corporate bonds | Level 2 | Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	41,636	27,670
Trading and Equity Securities		1,732
Recurring | Corporate bonds | Level 2 | Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	100,781	100,003
Trading and Equity Securities		2,460
Recurring | Corporate bonds | Level 3 | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Corporate bonds | Level 3 | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Corporate bonds | Level 3 | Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Trading and Equity Securities		0
Recurring | Corporate bonds | Level 3 | Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Trading and Equity Securities		0
Recurring | Corporate bonds | Level 3 | Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Trading and Equity Securities		0
Recurring | Corporate bonds | Level 3 | Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	250	250
Trading and Equity Securities		0
Recurring | Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	20,577	6,356
Recurring | Mortgage-backed securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Mortgage-backed securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	20,577	6,356
Recurring | Mortgage-backed securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Collateralized mortgage obligations | Government national mortgage association
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	86,152	80,356
Recurring | Collateralized mortgage obligations | Federal home loan mortgage corporation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	152,843	124,502
Recurring | Collateralized mortgage obligations | Federal national mortgage association
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	83,282	71,845
Recurring | Collateralized mortgage obligations | Level 1 | Government national mortgage association
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Collateralized mortgage obligations | Level 1 | Federal home loan mortgage corporation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Collateralized mortgage obligations | Level 1 | Federal national mortgage association
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Collateralized mortgage obligations | Level 2 | Government national mortgage association
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	86,152	80,356
Recurring | Collateralized mortgage obligations | Level 2 | Federal home loan mortgage corporation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	152,843	124,502
Recurring | Collateralized mortgage obligations | Level 2 | Federal national mortgage association
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	83,282	71,845
Recurring | Collateralized mortgage obligations | Level 3 | Government national mortgage association
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Collateralized mortgage obligations | Level 3 | Federal home loan mortgage corporation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Collateralized mortgage obligations | Level 3 | Federal national mortgage association
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	926	750
Recurring | Asset-backed securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Asset-backed securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)		0
Recurring | Asset-backed securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	926	750
Recurring | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	1,825,443	1,719,607
Recurring | Fixed maturities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	0	0
Recurring | Fixed maturities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	1,824,267	1,718,607
Recurring | Fixed maturities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale, at fair value (amortized cost $1,720,291 in 2020 and $1,659,760 in 2019; allowance for credit losses $5 in 2020 and $— in 2019)	1,176	1,000
Recurring | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities		4,087
Recurring | Redeemable preferred stocks | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities		4,087
Recurring | Redeemable preferred stocks | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities		0
Recurring | Redeemable preferred stocks | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities		0
Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities		299,203
Recurring | Equity securities | Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	16,320	16,295
Recurring | Equity securities | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	9,918	14,639
Recurring | Equity securities | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	36,556	57,330
Recurring | Equity securities | Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	32,061	29,935
Recurring | Equity securities | Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	24,549	27,285
Recurring | Equity securities | Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	17,109	127,780
Recurring | Equity securities | Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	69,577	19,265
Recurring | Equity securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities		298,608
Recurring | Equity securities | Level 1 | Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	16,320	16,295
Recurring | Equity securities | Level 1 | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	9,918	14,639
Recurring | Equity securities | Level 1 | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	36,556	57,330
Recurring | Equity securities | Level 1 | Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	32,061	29,935
Recurring | Equity securities | Level 1 | Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	24,549	27,285
Recurring | Equity securities | Level 1 | Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	17,109	127,780
Recurring | Equity securities | Level 1 | Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	69,577	19,265
Recurring | Equity securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities		0
Recurring | Equity securities | Level 2 | Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	0	0
Recurring | Equity securities | Level 2 | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	0	0
Recurring | Equity securities | Level 2 | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	0	0
Recurring | Equity securities | Level 2 | Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	0	0
Recurring | Equity securities | Level 2 | Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	0	0
Recurring | Equity securities | Level 2 | Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	0	0
Recurring | Equity securities | Level 2 | Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	0	0
Recurring | Equity securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities		595
Recurring | Equity securities | Level 3 | Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	0	0
Recurring | Equity securities | Level 3 | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	0	0
Recurring | Equity securities | Level 3 | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	0	0
Recurring | Equity securities | Level 3 | Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	0	0
Recurring | Equity securities | Level 3 | Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	0	0
Recurring | Equity securities | Level 3 | Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	0	0
Recurring | Equity securities | Level 3 | Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	0	0
Recurring | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	595	6,674
Recurring | Nonredeemable preferred stocks | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities		6,079
Recurring | Nonredeemable preferred stocks | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	0	0
Recurring | Nonredeemable preferred stocks | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading and Equity Securities	$ 595	$ 595