Basis of Presentation - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,501,573
Other comprehensive income (loss) before reclassifications	(109,539)	$ 22,330
Amounts reclassified from accumulated other comprehensive income (loss)	2,443	203
Cumulative effect of change in accounting principle (3)	(15,041)	0
Other comprehensive income (loss)	(122,137)	22,533
Ending balance	1,404,679	1,321,818
Other income tax expense (benefit)	(649)	(110)
Reclassification of unrealized gains on equity securities, net of tax	15,041	0
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	286,960	163,921
Cumulative effect of change in accounting principle (3)	(15,041)	0
Ending balance	164,823	186,454
Net Unrealized Investment Gains and Losses on Fixed Maturity and Equity Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	300,177	175,738
Other comprehensive income (loss) before reclassifications	(109,539)	22,330
Amounts reclassified from accumulated other comprehensive income (loss)	2,443	203
Cumulative effect of change in accounting principle (3)	(15,041)
Other comprehensive income (loss)	(122,137)	22,533
Ending balance	178,040	198,271
Pretax reclassification amounts from accumulated other comprehensive income (loss)	(3,092)	(313)
Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(13,217)	(11,817)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Cumulative effect of change in accounting principle (3)	0
Other comprehensive income (loss)	0	0
Ending balance	$ (13,217)	$ (11,817)