Investments - Reconciliation of Net Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 1,293,982
Ending balance	$ 1,390,441	$ 1,444,084	1,390,441	$ 1,444,084
Net Unrealized Investment Gains and Losses on Fixed Maturity and Equity Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	243,510	329,653	175,738	175,167
Ending balance	255,718	337,291	255,718	337,291
Net Unrealized Investment Gains and Losses on Fixed Maturity and Equity Securities | Fixed maturity securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	270,834	371,456	197,978	198,714
Change in net unrealized investment gains and losses	10,133	20,827	83,547	188,912
Reclassification of net realized investment (gains) losses to net income	2,587	(11,072)	2,029	(6,415)
Ending balance	283,554	381,211	283,554	381,211
Net Unrealized Investment Gains and Losses on Fixed Maturity and Equity Securities | Equity securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	10,631	8,183	4,963	2,649
Change in net unrealized investment gains and losses	1,933	(2,052)	7,905	4,846
Reclassification of net realized investment (gains) losses to net income	(165)	2,211	(469)	847
Ending balance	$ 12,399	$ 8,342	$ 12,399	$ 8,342