Investments - Reconciliation of Net Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,501,573
Cumulative effect of change in accounting principle	(15,041)	$ 0
Ending balance	1,404,679	1,321,818
Net Unrealized Investment Gains and Losses on Fixed Maturity and Equity Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	300,177	175,738
Cumulative effect of change in accounting principle	(15,041)
Ending balance	178,040	198,271
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	286,176	202,941
Change in net unrealized investment gains and losses	(67,285)	25,193
Reclassification of net investments (gains) losses to net income	2,443	203
Ending balance	$ 206,293	$ 228,337