Investments (Net Unrealized Gains Losses on Investments by Asset Class) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Net Unrealized Gains (Losses) on Investments	$ 274,418	$ 112,432	$ 117,033
Fixed Maturities | Available-for-sale Securities | OTTI
Gain (Loss) on Investments [Line Items]
Net Unrealized Gains (Losses) on Investments	1,609	4,883	5,196
Fixed Maturities | Available-for-sale Securities | All other
Gain (Loss) on Investments [Line Items]
Net Unrealized Gains (Losses) on Investments	279,749	59,755	59,930
Equity securities | Available-for-sale Securities
Gain (Loss) on Investments [Line Items]
Net Unrealized Gains (Losses) on Investments	2,368	366	(2,636)
Derivatives designated as cash flow hedges
Gain (Loss) on Investments [Line Items]
Net Unrealized Gains (Losses) on Investments	(17,678)	40,931	48,271
Affiliated notes
Gain (Loss) on Investments [Line Items]
Net Unrealized Gains (Losses) on Investments	4,782	5,056	5,064
Other investments
Gain (Loss) on Investments [Line Items]
Net Unrealized Gains (Losses) on Investments	$ 3,588	$ 1,441	$ 1,208