Summary of Investments - Realized Investments Gaines (Losses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Realized investments gains (losses), net	$ 46,403	$ 92,882	$ 83,710
Fixed maturity and short-term investments
Schedule of Investments [Line Items]
Realized investments gains (losses), net	13,443	87,108	46,027
Derivative instruments
Schedule of Investments [Line Items]
Realized investments gains (losses), net	8,969	(5,318)	5,840
Mortgage loans on real estate
Schedule of Investments [Line Items]
Realized investments gains (losses), net	13,583	10,972	31,841
Limited partnership and other corporation interests
Schedule of Investments [Line Items]
Realized investments gains (losses), net	10,383	0	0
Other
Schedule of Investments [Line Items]
Realized investments gains (losses), net	$ 25	$ 120	$ 2