Basis of Presentation - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 1,293,982
Other comprehensive income (loss) before reclassifications	$ 46,303	$ 87,809	68,633	$ 157,780
Amounts reclassified from accumulated other comprehensive income (loss)	(1,064)	(2,813)	(861)	(3,294)
Other comprehensive income	45,239	84,996	67,772	154,486
Ending balance	1,362,130	1,417,335	1,362,130	1,417,335
Other income tax expense (benefit)	574	1,514	464	1,773
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	186,454	232,863	163,921	163,373
Ending balance	231,693	317,859	231,693	317,859
Net Unrealized Investment Gains and Losses on Fixed Maturity and Equity Securities (1)(2)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	198,271	244,657	175,738	175,167
Other comprehensive income (loss) before reclassifications	46,303	87,809	68,633	157,780
Amounts reclassified from accumulated other comprehensive income (loss)	(1,064)	(2,813)	(861)	(3,294)
Other comprehensive income	45,239	84,996	67,772	154,486
Ending balance	243,510	329,653	243,510	329,653
Pretax reclassification amounts from accumulated other comprehensive income (loss)	1,638	4,327	1,325	5,067
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)