Basis of Presentation - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 1,501,573
Other comprehensive income (loss) before reclassifications	$ (49,165)	$ 9,786	(211,577)	$ 78,419
Amounts reclassified from accumulated other comprehensive income (loss)	(473)	2,422	2,399	1,561
Cumulative effect of change in accounting principle	0	0	(15,041)	0
Net current period other comprehensive income (loss)	(49,638)	12,208	(224,219)	79,980
Ending balance	1,304,256	1,390,441	1,304,256	1,390,441
Other income tax expense (benefit)	(1,304)	126	(840)	(638)
Reclassification of unrealized gains on equity securities, net of tax	0	0	15,041	0
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	112,379	231,693	286,960	163,921
Cumulative effect of change in accounting principle			(15,041)	0
Ending balance	62,741	243,901	62,741	243,901
Net Unrealized Investment Gains and Losses on Fixed Maturity and Equity Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	125,596	243,510	300,177	175,738
Other comprehensive income (loss) before reclassifications	(49,165)	9,786	(211,577)	78,419
Amounts reclassified from accumulated other comprehensive income (loss)	(473)	2,422	2,399	1,561
Cumulative effect of change in accounting principle	0		(15,041)
Net current period other comprehensive income (loss)	(49,638)	12,208	(224,219)	79,980
Ending balance	75,958	255,718	75,958	255,718
Pretax reclassification amounts from accumulated other comprehensive income (loss)	(3,726)	599	(2,401)	(3,037)
Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(13,217)	(11,817)	(13,217)	(11,817)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0	0
Cumulative effect of change in accounting principle	0		0
Net current period other comprehensive income (loss)	0	0	0	0
Ending balance	$ (13,217)	$ (11,817)	$ (13,217)	$ (11,817)