Basis of Presentation - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 1,293,982
Other comprehensive income (loss) before reclassifications	$ 9,786	$ 9,912	78,419	$ 167,692
Amounts reclassified from accumulated other comprehensive income (loss)	2,422	(2,274)	1,561	(5,568)
Other comprehensive income	12,208	7,638	79,980	162,124
Ending balance	1,390,441	1,444,084	1,390,441	1,444,084
Other income tax expense (benefit)	(1,304)	1,225	(840)	2,998
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	231,693	317,859	163,921	163,373
Ending balance	243,901	325,497	243,901	325,497
Net Unrealized Investment Gains and Losses on Fixed Maturity and Equity Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	243,510	329,653	175,738	175,167
Other comprehensive income (loss) before reclassifications	9,786	9,912	78,419	167,692
Amounts reclassified from accumulated other comprehensive income (loss)	2,422	(2,274)	1,561	(5,568)
Other comprehensive income	12,208	7,638	79,980	162,124
Ending balance	255,718	337,291	255,718	337,291
Pretax reclassification amounts from accumulated other comprehensive income (loss)	(3,726)	3,499	(2,401)	8,566
Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(11,817)	(11,794)	(11,817)	(11,794)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0	0
Other comprehensive income	0	0	0	0
Ending balance	$ (11,817)	$ (11,794)	$ (11,817)	$ (11,794)