Fair Value of Financial Instruments - Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	$ 1,968,949	$ 1,837,332
Trading securities	$ 6,228	7,383
Fair value assumptions, percentage of securities assessed by third-party pricing service	94.00%
Fair Value, Recurring Measurements [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Trading securities	$ 6,228	7,383
Assets, fair value disclosure, recurring	4,548,049	4,132,668
Liabilities, fair value disclosure, recurring	2,572,872	2,287,953
Fair Value, Recurring Measurements [Member] | U.S. Government and Agencies [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	11,813	10,474
Fair Value, Recurring Measurements [Member] | Foreign Government [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	144,489	129,306
Fair Value, Recurring Measurements [Member] | States and Political Subdivisions [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	56,127	45,724
Fair Value, Recurring Measurements [Member] | Corporates [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	1,376,282	1,325,373
Fair Value, Recurring Measurements [Member] | Residential Mortgage-backed Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	122,958	99,551
Fair Value, Recurring Measurements [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	135,392	109,443
Fair Value, Recurring Measurements [Member] | Fixed-maturity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	1,927,842	1,792,438
Fair Value, Recurring Measurements [Member] | Other Asset-backed Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	80,781	72,567
Fair Value, Recurring Measurements [Member] | Equity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	41,107	44,894
Fair Value, Recurring Measurements [Member] | Separate Accounts Assets, Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Separate accounts assets	2,572,872	2,287,953
Fair Value, Recurring Measurements [Member] | Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Trading securities	1,428	51
Assets, fair value disclosure, recurring	43,694	42,720
Fair Value, Recurring Measurements [Member] | Level 1 [Member] | Corporates [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	3,240	3,113
Fair Value, Recurring Measurements [Member] | Level 1 [Member] | Fixed-maturity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	3,240	3,113
Fair Value, Recurring Measurements [Member] | Level 1 [Member] | Equity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	39,026	39,556
Fair Value, Recurring Measurements [Member] | Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Trading securities	4,800	7,332
Assets, fair value disclosure, recurring	4,503,788	4,081,786
Liabilities, fair value disclosure, recurring	2,572,872	2,287,953
Fair Value, Recurring Measurements [Member] | Level 2 [Member] | U.S. Government and Agencies [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	11,813	10,474
Fair Value, Recurring Measurements [Member] | Level 2 [Member] | Foreign Government [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	144,489	129,306
Fair Value, Recurring Measurements [Member] | Level 2 [Member] | States and Political Subdivisions [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	56,127	45,724
Fair Value, Recurring Measurements [Member] | Level 2 [Member] | Corporates [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	1,373,039	1,322,257
Fair Value, Recurring Measurements [Member] | Level 2 [Member] | Residential Mortgage-backed Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	122,544	98,966
Fair Value, Recurring Measurements [Member] | Level 2 [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	135,392	109,443
Fair Value, Recurring Measurements [Member] | Level 2 [Member] | Fixed-maturity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	1,924,185	1,781,245
Fair Value, Recurring Measurements [Member] | Level 2 [Member] | Other Asset-backed Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	80,781	65,075
Fair Value, Recurring Measurements [Member] | Level 2 [Member] | Equity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	1,931	5,256
Fair Value, Recurring Measurements [Member] | Level 2 [Member] | Separate Accounts Assets, Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Separate accounts assets	2,572,872	2,287,953
Fair Value, Recurring Measurements [Member] | Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value disclosure, recurring	567	8,162
Fair Value, Recurring Measurements [Member] | Level 3 [Member] | Corporates [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	3	3
Fair Value, Recurring Measurements [Member] | Level 3 [Member] | Residential Mortgage-backed Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	414	585
Fair Value, Recurring Measurements [Member] | Level 3 [Member] | Fixed-maturity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	417	8,080
Fair Value, Recurring Measurements [Member] | Level 3 [Member] | Other Asset-backed Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value		7,492
Fair Value, Recurring Measurements [Member] | Level 3 [Member] | Equity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale securities, fair value	150	82
Fair Value, Recurring Measurements [Member] | Separate Accounts Liabilities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Separate accounts liabilities	2,572,872	2,287,953
Fair Value, Recurring Measurements [Member] | Separate Accounts Liabilities [Member] | Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Separate accounts liabilities	$ 2,572,872	$ 2,287,953