Investments - Net Unrealized Investment Gains and Losses on Fixed Maturities and Equity Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,293,982	$ 1,264,661
Ending balance	1,501,573	1,293,982	$ 1,264,661
Unrealized Gains and Losses on Fixed Maturity and Equity Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	175,738	175,167	297,554
Ending balance	300,177	175,738	175,167
Unrealized Gains and Losses on Fixed Maturity and Equity Securities | Fixed maturity securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	197,978	198,714	336,604
Change in unrealized investment gains and losses	69,989	3,024	(131,202)
Reclassification of net realized investment (gains) losses to net income	(5,764)	3,760	6,688
Ending balance	273,731	197,978	198,714
Unrealized Gains and Losses on Fixed Maturity and Equity Securities | Equity securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	4,963	2,649	6,988
Change in unrealized investment gains and losses	10,084	1,919	(3,443)
Reclassification of net realized investment (gains) losses to net income	2,602	(395)	896
Ending balance	$ 12,445	$ 4,963	$ 2,649