Investments - Reconciliation of Net Unrealized Investment Gains (Losses) (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
Ending balance	$ 1,362,130	$ 1,417,335	1,362,130	$ 1,417,335
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
Ending balance	243,510	329,653	243,510	329,653
Net Unrealized Investment Gains and Losses on Fixed Maturity and Equity Securities (1)(2) | Fixed maturity securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	219,385	276,381	197,978	198,714
Change in net unrealized investment gains and losses	51,523	89,744	73,414	168,085
Reclassification of net realized investment gains to net income	(74)	5,331	(558)	4,657
Ending balance	270,834	371,456	270,834	371,456
Net Unrealized Investment Gains and Losses on Fixed Maturity and Equity Securities (1)(2) | Equity securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	8,952	5,030	4,963	2,649
Change in net unrealized investment gains and losses	2,670	4,710	5,972	6,898
Reclassification of net realized investment gains to net income	(991)	(1,557)	(304)	(1,364)
Ending balance	$ 10,631	$ 8,183	$ 10,631	$ 8,183