Investments (Net Unrealized Gains Losses on Investments by Asset Class) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Gain (Loss) on Investments [Line Items]
Net Unrealized Gains (Losses) on Investments	$ 115,720	$ 274,418
Fixed maturities | Available-for-sale | OTTI
Gain (Loss) on Investments [Line Items]
Net Unrealized Gains (Losses) on Investments	697	1,609
Fixed maturities | Available-for-sale | All other
Gain (Loss) on Investments [Line Items]
Net Unrealized Gains (Losses) on Investments	150,981	279,749
Equity securities | Available-for-sale
Gain (Loss) on Investments [Line Items]
Net Unrealized Gains (Losses) on Investments	0	2,368
Derivatives designated as cash flow hedges
Gain (Loss) on Investments [Line Items]
Net Unrealized Gains (Losses) on Investments	(40,540)	(17,678)
Affiliated notes
Gain (Loss) on Investments [Line Items]
Net Unrealized Gains (Losses) on Investments	1,844	4,782
Other investments
Gain (Loss) on Investments [Line Items]
Net Unrealized Gains (Losses) on Investments	$ 2,738	$ 3,588