Investments - Schedule of Fair Value of Financial Instruments Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities
Fair value of financial instruments	$ 872,444	$ 834,531
U.S. Government and Agency Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	242,259	235,891
Foreign Government
Schedule of Available-for-sale Securities
Fair value of financial instruments	2,012	2,036
States, Municipalities and Political Subdivisions
Schedule of Available-for-sale Securities
Fair value of financial instruments	185,868	201,512
Public Utilities
Schedule of Available-for-sale Securities
Fair value of financial instruments	24,169	20,257
Corporate Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	287,247	287,562
Asset-backed Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	44,903	14,905
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fair value of financial instruments	687	692
Fixed Maturities
Schedule of Available-for-sale Securities
Fair value of financial instruments	787,145	762,855
Public Utilities
Schedule of Available-for-sale Securities
Fair value of financial instruments	1,603	1,702
Common Stock
Schedule of Available-for-sale Securities
Fair value of financial instruments	27,577	27,902
Nonredeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fair value of financial instruments	1,739	1,767
Equity Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	77,155	63,295
Other Long-term Investments
Schedule of Available-for-sale Securities
Fair value of financial instruments	8,144	8,381
Mutual Fund
Schedule of Available-for-sale Securities
Fair value of financial instruments	46,236	31,924
Level 1
Schedule of Available-for-sale Securities
Fair value of financial instruments	78,142	64,287
Level 1 | U.S. Government and Agency Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 1 | Foreign Government
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 1 | States, Municipalities and Political Subdivisions
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 1 | Public Utilities
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 1 | Corporate Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 1 | Asset-backed Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 1 | Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fair value of financial instruments	687	692
Level 1 | Fixed Maturities
Schedule of Available-for-sale Securities
Fair value of financial instruments	687	692
Level 1 | Public Utilities
Schedule of Available-for-sale Securities
Fair value of financial instruments	1,603	1,702
Level 1 | Common Stock
Schedule of Available-for-sale Securities
Fair value of financial instruments	27,577	27,902
Level 1 | Nonredeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fair value of financial instruments	1,739	1,767
Level 1 | Equity Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	77,155	63,295
Level 1 | Other Long-term Investments
Schedule of Available-for-sale Securities
Fair value of financial instruments	300	300
Level 1 | Mutual Fund
Schedule of Available-for-sale Securities
Fair value of financial instruments	46,236	31,924
Level 2
Schedule of Available-for-sale Securities
Fair value of financial instruments	793,648	769,610
Level 2 | U.S. Government and Agency Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	242,259	235,891
Level 2 | Foreign Government
Schedule of Available-for-sale Securities
Fair value of financial instruments	2,012	2,036
Level 2 | States, Municipalities and Political Subdivisions
Schedule of Available-for-sale Securities
Fair value of financial instruments	185,868	201,512
Level 2 | Public Utilities
Schedule of Available-for-sale Securities
Fair value of financial instruments	24,169	20,257
Level 2 | Corporate Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	287,247	287,562
Level 2 | Asset-backed Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	44,903	14,905
Level 2 | Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 2 | Fixed Maturities
Schedule of Available-for-sale Securities
Fair value of financial instruments	786,458	762,163
Level 2 | Public Utilities
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 2 | Common Stock
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 2 | Nonredeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 2 | Equity Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 2 | Other Long-term Investments
Schedule of Available-for-sale Securities
Fair value of financial instruments	7,190	7,447
Level 2 | Mutual Fund
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 3
Schedule of Available-for-sale Securities
Fair value of financial instruments	654	634
Level 3 | U.S. Government and Agency Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 3 | Foreign Government
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 3 | States, Municipalities and Political Subdivisions
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 3 | Public Utilities
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 3 | Corporate Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 3 | Asset-backed Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 3 | Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 3 | Fixed Maturities
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 3 | Public Utilities
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 3 | Common Stock
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 3 | Nonredeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 3 | Equity Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	0	0
Level 3 | Other Long-term Investments
Schedule of Available-for-sale Securities
Fair value of financial instruments	654	634
Level 3 | Mutual Fund
Schedule of Available-for-sale Securities
Fair value of financial instruments	$ 0	$ 0