Basis of Presentation - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 1,501,573
Other comprehensive income (loss) before reclassifications	$ (52,873)	$ 46,303	(162,412)	$ 68,633
Amounts reclassified from accumulated other comprehensive income (loss)	429	(1,064)	2,872	(861)
Cumulative effect of change in accounting principle (3)	0	0	(15,041)	0
Other comprehensive income (loss)	(52,444)	45,239	(174,581)	67,772
Ending balance	1,350,441	1,362,130	1,350,441	1,362,130
Other income tax expense (benefit)	(115)	574	(764)	464
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	164,823	186,454	286,960	163,921
Cumulative effect of change in accounting principle (3)			(15,041)	0
Ending balance	112,379	231,693	112,379	231,693
Net Unrealized Investment Gains and Losses on Fixed Maturity and Equity Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	178,040	198,271	300,177	175,738
Other comprehensive income (loss) before reclassifications	(52,873)	46,303	(162,412)	68,633
Amounts reclassified from accumulated other comprehensive income (loss)	429	(1,064)	2,872	(861)
Cumulative effect of change in accounting principle (3)	0		(15,041)
Other comprehensive income (loss)	(52,444)	45,239	(174,581)	67,772
Ending balance	125,596	243,510	125,596	243,510
Pretax reclassification amounts from accumulated other comprehensive income (loss)	(544)	1,638	(3,636)	1,325
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 (3)	0		0
Other comprehensive income (loss)	0	0	0	0
Ending balance	$ (13,217)	$ (11,817)	$ (13,217)	$ (11,817)