Investments - Schedule of Fair Value of Financial Instruments Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities
Fair value of financial instruments	$ 788,581	$ 528,647
U.S. Government and Agency Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	203,456	149,952
Foreign Government
Schedule of Available-for-sale Securities
Fair value of financial instruments	2,057	2,061
States, Municipalities and Political Subdivisions
Schedule of Available-for-sale Securities
Fair value of financial instruments	196,147	169,112
Public Utilities
Schedule of Available-for-sale Securities
Fair value of financial instruments	19,755	7,730
Corporate Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	279,593	164,536
Asset-backed Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	16,714
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fair value of financial instruments	1,063	1,125
Fixed Maturities
Schedule of Available-for-sale Securities
Fair value of financial instruments	718,785	494,516
Public Utilities
Schedule of Available-for-sale Securities
Fair value of financial instruments	1,642	1,507
Common Stock
Schedule of Available-for-sale Securities
Fair value of financial instruments	27,445	24,048
Nonredeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fair value of financial instruments	2,745	2,843
Equity Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	61,639	28,398
Other Long-term Investments
Schedule of Available-for-sale Securities
Fair value of financial instruments	8,157	5,733
Mutual Fund
Schedule of Available-for-sale Securities
Fair value of financial instruments	29,807
Level 1
Schedule of Available-for-sale Securities
Fair value of financial instruments	63,002	29,823
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
Level 1 | Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fair value of financial instruments	1,063	1,125
Level 1 | Fixed Maturities
Schedule of Available-for-sale Securities
Fair value of financial instruments	1,063	1,125
Level 1 | Public Utilities
Schedule of Available-for-sale Securities
Fair value of financial instruments	1,642	1,507
Level 1 | Common Stock
Schedule of Available-for-sale Securities
Fair value of financial instruments	27,445	24,048
Level 1 | Nonredeemable Preferred Stocks
Schedule of Available-for-sale Securities
Fair value of financial instruments	2,745	2,843
Level 1 | Equity Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	61,639	28,398
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	29,807
Level 2
Schedule of Available-for-sale Securities
Fair value of financial instruments	724,895	497,126
Level 2 | U.S. Government and Agency Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	203,456	149,952
Level 2 | Foreign Government
Schedule of Available-for-sale Securities
Fair value of financial instruments	2,057	2,061
Level 2 | States, Municipalities and Political Subdivisions
Schedule of Available-for-sale Securities
Fair value of financial instruments	196,147	169,112
Level 2 | Public Utilities
Schedule of Available-for-sale Securities
Fair value of financial instruments	19,755	7,730
Level 2 | Corporate Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	279,593	164,536
Level 2 | Asset-backed Securities
Schedule of Available-for-sale Securities
Fair value of financial instruments	16,714
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	717,722	493,391
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	7,173	3,735
Level 2 | Mutual Fund
Schedule of Available-for-sale Securities
Fair value of financial instruments	0
Level 3
Schedule of Available-for-sale Securities
Fair value of financial instruments	684	1,698
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
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	684	$ 1,698
Level 3 | Mutual Fund
Schedule of Available-for-sale Securities
Fair value of financial instruments	$ 0