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	$ 112,432	$ 117,033	$ 349,910
Fixed maturity securities on which an OTTI loss has been recognized | Available-for-sale Securities
Investment [Line Items]
Net unrealized gains (losses) on investments	4,883	5,196	5,333
Fixed maturities, available-for-sale | Available-for-sale Securities
Investment [Line Items]
Net unrealized gains (losses) on investments	59,755	59,930	322,358
Equity securities, available-for-sale: | Available-for-sale Securities
Investment [Line Items]
Net unrealized gains (losses) on investments	366	(2,636)	619
Derivatives designated as cash flow hedges
Investment [Line Items]
Net unrealized gains (losses) on investments	40,931	48,271	11,585
Other investments
Investment [Line Items]
Net unrealized gains (losses) on investments	$ 6,497	$ 6,272	$ 10,015