Fair Value of Financial Instruments (Financial Instruments Measured at Fair Value) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 1,730,563	$ 1,749,488
Trading securities	16,409	13,240
Equity securities	269,843	248,361
U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	26,691	27,418
U.S. government agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	189,541	214,682
States, municipalities and political subdivisions \| General obligations \| Midwest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	88,679	95,212
States, municipalities and political subdivisions \| General obligations \| Northeast
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	32,132	37,655
States, municipalities and political subdivisions \| General obligations \| South
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	115,780	113,911
States, municipalities and political subdivisions \| General obligations \| West
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	109,867	107,841
States, municipalities and political subdivisions \| Special revenue \| Midwest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	143,129	140,764
States, municipalities and political subdivisions \| Special revenue \| Northeast
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	63,321	61,948
States, municipalities and political subdivisions \| Special revenue \| South
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	232,936	235,809
States, municipalities and political subdivisions \| Special revenue \| West
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	145,342	142,920
Foreign bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	5,047	9,716
Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	63,208	56,059
Corporate bonds \| Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	28,653	28,648
Corporate bonds \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	54,278	53,085
Corporate bonds \| Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	48,799	53,646
Corporate bonds \| Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	14,266	16,658
Corporate bonds \| Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	26,220	26,176
Corporate bonds \| Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	92,573	79,349
Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	7,315	7,424
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	3,536	3,495
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,730,563	1,749,488
Trading securities	16,409	13,240
Equity securities	269,843	248,361
Short-term investments	175	175
Money market accounts	10,039	3,275
Corporate-owned life insurance	5,708	4,907
Total assets measured at fair value	2,032,737	2,019,446
Recurring \| Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	15,330	15,949
Recurring \| Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	12,412	10,975
Recurring \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	61,773	53,536
Recurring \| Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	26,860	24,465
Recurring \| Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	24,234	22,286
Recurring \| Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	16,370	13,944
Recurring \| Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	108,216	101,555
Recurring \| U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	26,691	27,418
Trading securities	746
Recurring \| U.S. government agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	189,541	214,682
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 securities	88,679	95,212
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 securities	32,132	37,655
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 securities	115,780	113,911
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 securities	109,867	107,841
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 securities	143,129	140,764
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 securities	63,321	61,948
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 securities	232,936	235,809
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 securities	145,342	142,920
Recurring \| Foreign bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	5,047	9,716
Recurring \| Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	63,208	56,059
Recurring \| Corporate bonds \| Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	28,653	28,648
Recurring \| Corporate bonds \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	54,278	53,085
Trading securities	399	397
Recurring \| Corporate bonds \| Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	48,799	53,646
Trading securities	1,822	1,599
Recurring \| Corporate bonds \| Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	14,266	16,658
Trading securities	4,809	3,236
Recurring \| Corporate bonds \| Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	26,220	26,176
Trading securities	3,320	3,028
Recurring \| Corporate bonds \| Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	92,573	79,349
Trading securities	1,840	2,231
Recurring \| Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	7,315	7,424
Recurring \| Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	3,536	3,495
Recurring \| Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	4,648	5,651
Recurring \| Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	3,473	2,749
Recurring \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Trading securities	3,473	2,749
Equity securities	269,248	247,766
Short-term investments	175	175
Money market accounts	10,039	3,275
Corporate-owned life insurance	0	0
Total assets measured at fair value	282,935	253,965
Recurring \| Level 1 \| Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	15,330	15,949
Recurring \| Level 1 \| Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	12,412	10,975
Recurring \| Level 1 \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	61,773	53,536
Recurring \| Level 1 \| Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	26,860	24,465
Recurring \| Level 1 \| Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	24,234	22,286
Recurring \| Level 1 \| Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	16,370	13,944
Recurring \| Level 1 \| Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	108,216	101,555
Recurring \| Level 1 \| U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Trading securities	0
Recurring \| Level 1 \| U.S. government agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 1 \| States, municipalities and political subdivisions \| General obligations \| Midwest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 1 \| States, municipalities and political subdivisions \| General obligations \| Northeast
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 1 \| States, municipalities and political subdivisions \| General obligations \| South
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 1 \| States, municipalities and political subdivisions \| General obligations \| West
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 1 \| States, municipalities and political subdivisions \| Special revenue \| Midwest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 1 \| States, municipalities and political subdivisions \| Special revenue \| Northeast
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 1 \| States, municipalities and political subdivisions \| Special revenue \| South
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 1 \| States, municipalities and political subdivisions \| Special revenue \| West
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 1 \| Foreign bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 1 \| Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 1 \| Corporate bonds \| Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 1 \| Corporate bonds \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Trading securities	0	0
Recurring \| Level 1 \| Corporate bonds \| Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Trading securities	0	0
Recurring \| Level 1 \| Corporate bonds \| Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Trading securities	0	0
Recurring \| Level 1 \| Corporate bonds \| Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Trading securities	0	0
Recurring \| Level 1 \| Corporate bonds \| Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Trading securities	0	0
Recurring \| Level 1 \| Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 1 \| Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 1 \| Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	4,053	5,056
Recurring \| Level 1 \| Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	3,473	2,749
Recurring \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,729,612	1,748,572
Trading securities	12,936	10,491
Equity securities	0	0
Short-term investments	0	0
Money market accounts	0	0
Corporate-owned life insurance	5,708	4,907
Total assets measured at fair value	1,748,256	1,763,970
Recurring \| Level 2 \| Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Recurring \| Level 2 \| Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Recurring \| Level 2 \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Recurring \| Level 2 \| Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Recurring \| Level 2 \| Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Recurring \| Level 2 \| Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Recurring \| Level 2 \| Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Recurring \| Level 2 \| U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	26,691	27,418
Trading securities	746
Recurring \| Level 2 \| U.S. government agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	189,541	214,682
Recurring \| Level 2 \| States, municipalities and political subdivisions \| General obligations \| Midwest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	88,679	95,212
Recurring \| Level 2 \| States, municipalities and political subdivisions \| General obligations \| Northeast
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	32,132	37,655
Recurring \| Level 2 \| States, municipalities and political subdivisions \| General obligations \| South
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	115,780	113,911
Recurring \| Level 2 \| States, municipalities and political subdivisions \| General obligations \| West
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	109,867	107,841
Recurring \| Level 2 \| States, municipalities and political subdivisions \| Special revenue \| Midwest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	143,129	140,764
Recurring \| Level 2 \| States, municipalities and political subdivisions \| Special revenue \| Northeast
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	63,321	61,948
Recurring \| Level 2 \| States, municipalities and political subdivisions \| Special revenue \| South
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	232,936	235,809
Recurring \| Level 2 \| States, municipalities and political subdivisions \| Special revenue \| West
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	145,342	142,920
Recurring \| Level 2 \| Foreign bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	5,047	9,716
Recurring \| Level 2 \| Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	63,208	56,059
Recurring \| Level 2 \| Corporate bonds \| Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	28,653	28,648
Recurring \| Level 2 \| Corporate bonds \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	54,278	53,085
Trading securities	399	397
Recurring \| Level 2 \| Corporate bonds \| Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	48,799	53,646
Trading securities	1,822	1,599
Recurring \| Level 2 \| Corporate bonds \| Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	14,266	16,658
Trading securities	4,809	3,236
Recurring \| Level 2 \| Corporate bonds \| Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	26,220	26,176
Trading securities	3,320	3,028
Recurring \| Level 2 \| Corporate bonds \| Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	92,323	79,099
Trading securities	1,840	2,231
Recurring \| Level 2 \| Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	7,315	7,424
Recurring \| Level 2 \| Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	2,835	2,829
Recurring \| Level 2 \| Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Recurring \| Level 2 \| Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Recurring \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	951	916
Trading securities	0	0
Equity securities	595	595
Short-term investments	0	0
Money market accounts	0	0
Corporate-owned life insurance	0	0
Total assets measured at fair value	1,546	1,511
Recurring \| Level 3 \| Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Recurring \| Level 3 \| Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Recurring \| Level 3 \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Recurring \| Level 3 \| Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Recurring \| Level 3 \| Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Recurring \| Level 3 \| Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Recurring \| Level 3 \| Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Recurring \| Level 3 \| U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Trading securities	0
Recurring \| Level 3 \| U.S. government agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 3 \| States, municipalities and political subdivisions \| General obligations \| Midwest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 3 \| States, municipalities and political subdivisions \| General obligations \| Northeast
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 3 \| States, municipalities and political subdivisions \| General obligations \| South
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 3 \| States, municipalities and political subdivisions \| General obligations \| West
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 3 \| States, municipalities and political subdivisions \| Special revenue \| Midwest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 3 \| States, municipalities and political subdivisions \| Special revenue \| Northeast
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 3 \| States, municipalities and political subdivisions \| Special revenue \| South
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 3 \| States, municipalities and political subdivisions \| Special revenue \| West
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 3 \| Foreign bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 3 \| Public utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 3 \| Corporate bonds \| Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 3 \| Corporate bonds \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Trading securities	0	0
Recurring \| Level 3 \| Corporate bonds \| Consumer goods and services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Trading securities	0	0
Recurring \| Level 3 \| Corporate bonds \| Health care
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Trading securities	0	0
Recurring \| Level 3 \| Corporate bonds \| Technology, media and telecommunications
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Trading securities	0	0
Recurring \| Level 3 \| Corporate bonds \| Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	250	250
Trading securities	0	0
Recurring \| Level 3 \| Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Recurring \| Level 3 \| Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	701	666
Recurring \| Level 3 \| Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	595	595
Recurring \| Level 3 \| Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Federal Home Loan Mortgage Corporation (FHLMC) Insured Loans \| Collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	108,709	107,623
Federal Home Loan Mortgage Corporation (FHLMC) Insured Loans \| Recurring \| Collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	108,709	107,623
Federal Home Loan Mortgage Corporation (FHLMC) Insured Loans \| Recurring \| Level 1 \| Collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Federal Home Loan Mortgage Corporation (FHLMC) Insured Loans \| Recurring \| Level 2 \| Collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	108,709	107,623
Federal Home Loan Mortgage Corporation (FHLMC) Insured Loans \| Recurring \| Level 3 \| Collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Federal National Mortgage Association (FNMA) Insured Loans \| Collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	53,153	52,748
Federal National Mortgage Association (FNMA) Insured Loans \| Recurring \| Collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	53,153	52,748
Federal National Mortgage Association (FNMA) Insured Loans \| Recurring \| Level 1 \| Collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Federal National Mortgage Association (FNMA) Insured Loans \| Recurring \| Level 2 \| Collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	53,153	52,748
Federal National Mortgage Association (FNMA) Insured Loans \| Recurring \| Level 3 \| Collateralized mortgage obligations

Fair Value of Financial Instruments (Financial Instruments Measured at Fair Value) (Details) - USD ($)
$ in Thousands

Mar. 31, 2019

Dec. 31, 2018

Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]

Available-for-sale securities

$ 1,730,563

$ 1,749,488

Trading securities

16,409

13,240

Equity securities

269,843