Investments (Net Unrealized Gains Losses on Investments by Asset Class) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Investment [Line Items]
Net unrealized gains (losses) on investments	$ 399,372	$ 117,033
Derivatives designated as cash flow hedges
Investment [Line Items]
Net unrealized gains (losses) on investments	31,295	48,271
Securities Related to Other Than Temporary Impairments
Investment [Line Items]
Net unrealized gains (losses) on investments	4,469	5,196
Fixed maturities | Available-for-sale Securities
Investment [Line Items]
Net unrealized gains (losses) on investments	351,169	59,930
Equity securities, available-for-sale | Available-for-sale Securities
Investment [Line Items]
Net unrealized gains (losses) on investments	718	(2,636)
Other investments
Investment [Line Items]
Net unrealized gains (losses) on investments	$ 11,721	$ 6,272