Summary of Significant Accounting Policies - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,293,982	$ 1,264,661
Other comprehensive income (loss) before reclassifications	71,978	3,912	$ (113,644)
Amounts reclassified from accumulated other comprehensive income (loss)	3,161	(3,364)	(7,584)
Reclassification of deferred taxes	47,900
Net current period other comprehensive income (loss)	123,039
Other comprehensive income (loss)	75,139	548	(121,228)
Ending balance	1,501,573	1,293,982	1,264,661
Stranded deferred tax assets	(2,134)
Stranded deferred tax liabilities	50,034
Retained earnings
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	1,155,732	1,116,277	1,065,318
Reclassification of deferred taxes	(47,900)	0	0
Ending balance	1,231,177	1,155,732	1,116,277
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	163,921	163,373	284,601
Reclassification of deferred taxes	47,900	0	0
Ending balance	286,960	163,921	163,373
Unrealized Gains and Losses on Fixed Maturity and Equity Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	175,738	175,167	297,554
Other comprehensive income (loss) before reclassifications	71,244	3,935	(114,803)
Amounts reclassified from accumulated other comprehensive income (loss)	3,161	(3,364)	(7,584)
Reclassification of deferred taxes	50,034
Net current period other comprehensive income (loss)	124,439
Other comprehensive income (loss)		571	(122,387)
Ending balance	300,177	175,738	175,167
Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(11,817)	(11,794)	(12,953)
Other comprehensive income (loss) before reclassifications	734	(23)	1,159
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0
Reclassification of deferred taxes	(2,134)
Net current period other comprehensive income (loss)	(1,400)
Other comprehensive income (loss)		(23)	1,159
Ending balance	$ (13,217)	$ (11,817)	$ (11,794)