Summary of Investments - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Limited partnership and other corporation interests	$ 45,540,000	$ 34,895,000
Carrying value in relation to the activities of the VIEs	45,540,000	32,444,000
Maximum loss exposure of VIE	45,540,000	32,444,000
Securities loaned to third parties at amortized cost	0	0
Total fixed maturity investments
Schedule of Investments [Line Items]
Change in net unrealized holding loss	$ 246,465,000
Change in net realized holding loss (as a percent)	63.00%
Decrease in unrealized and other than temporary impairment losses, less than 12 months	$ 206,316,000
Increase (decrease) in greater than twelve months unrealized losses and other-than-temporary impairment losses	(40,149,000)
Unrealized loss and OTTI, 12 months or longer	$ 114,292,000	154,441,000
Corporate debt securities
Schedule of Investments [Line Items]
Unrealized losses and OTTI over twelve months on securities (as a percent)	80.00%
Unrealized loss and OTTI, 12 months or longer	$ 90,809,000	$ 138,711,000
Asset-backed and Commercial Mortgage Backed Securities
Schedule of Investments [Line Items]
Unrealized losses and OTTI over twelve months on securities (as a percent)	15.00%
External Credit Rating, Non Investment Grade | Corporate debt securities
Schedule of Investments [Line Items]
Unrealized loss and OTTI, 12 months or longer	$ 6,244,000