Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	$ 5,287,360	$ 5,151,987
Equity securities, at fair value (cost 2019 - $4,016; 2018 - $3,993)	4,057	3,932
Other	205,444	96,403
Real estate acquired	11,639	14,535
Total assets	5,508,500	5,266,857
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	205,444	96,403
Real estate acquired	0	0
Total assets	251,980	142,599
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	0	0
Real estate acquired	0	0
Total assets	5,244,881	5,109,710
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	0	0
Real estate acquired	11,639	14,535
Total assets	11,639	14,548
Total fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	5,287,360	5,151,987
Total fixed income securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	42,479	42,264
Total fixed income securities | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	5,244,881	5,109,710
Total fixed income securities | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	0	13
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	168,142	167,176
U.S. Treasury securities and obligations of U.S. government corporations and agencies | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	42,479	42,264
U.S. Treasury securities and obligations of U.S. government corporations and agencies | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	125,663	124,912
U.S. Treasury securities and obligations of U.S. government corporations and agencies | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	0	0
Obligations of U.S. states and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	1,667,238	1,720,100
Obligations of U.S. states and political subdivisions | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	0	0
Obligations of U.S. states and political subdivisions | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	1,667,238	1,720,087
Obligations of U.S. states and political subdivisions | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	0	13
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	2,456,069	2,400,762
Corporate debt securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	0	0
Corporate debt securities | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	2,456,069	2,400,762
Corporate debt securities | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	0	0
Asset backed securities (“ABS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	218,481	112,033
Asset backed securities (“ABS”) | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	0	0
Asset backed securities (“ABS”) | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	218,481	112,033
Asset backed securities (“ABS”) | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	0	0
Residential mortgage backed securities (“RMBS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	180,485	178,961
Residential mortgage backed securities (“RMBS”) | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	0	0
Residential mortgage backed securities (“RMBS”) | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	180,485	178,961
Residential mortgage backed securities (“RMBS”) | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	0	0
Commercial mortgage backed securities (“CMBS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	269,283	267,660
Commercial mortgage backed securities (“CMBS”) | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	0	0
Commercial mortgage backed securities (“CMBS”) | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	269,283	267,660
Commercial mortgage backed securities (“CMBS”) | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	0	0
Collateralized loan obligations (“CLO”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	327,662	305,295
Collateralized loan obligations (“CLO”) | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	0	0
Collateralized loan obligations (“CLO”) | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	327,662	305,295
Collateralized loan obligations (“CLO”) | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income, available-for-sale, at fair value (amortized cost 2019 - $5,229,535; 2018 - $5,196,784)	0	0
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value (cost 2019 - $4,016; 2018 - $3,993)	4,057	3,932
Equity securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value (cost 2019 - $4,016; 2018 - $3,993)	4,057	3,932
Equity securities | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value (cost 2019 - $4,016; 2018 - $3,993)	0	0
Equity securities | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value (cost 2019 - $4,016; 2018 - $3,993)	$ 0	$ 0