Investments (Net Unrealized Gains Losses on Investments by Asset Class) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment [Line Items]
Net unrealized gains (losses) on investments	$ (443,575)	$ 107,460	$ 198,923
Securities related to other than temporary impairments
Investment [Line Items]
Net unrealized gains (losses) on investments	(1,261)	9	1
Fixed maturities
Investment [Line Items]
Net unrealized gains (losses) on investments	(454,274)	90,637	191,339
Equity securities, available-for-sale
Investment [Line Items]
Net unrealized gains (losses) on investments	(347)	3	3
Affiliated notes
Investment [Line Items]
Net unrealized gains (losses) on investments	1,181	1,660	2,351
Derivatives designated as cash flow hedges
Investment [Line Items]
Net unrealized gains (losses) on investments	11,745	14,847	4,839
Other investments
Investment [Line Items]
Net unrealized gains (losses) on investments	$ (619)	$ 304	$ 390