Investments (Net Unrealized Gains Losses on Investments by Asset Class) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	$ (571,423)	$ (59,412)
Fixed maturities | Available-for-sale | OTTI
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	(1,034)	12,311
Fixed maturities | Available-for-sale | All other
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	(548,232)	(46,791)
Equity Securities | Available-for-sale
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	0	4
Derivatives designated as cash flow hedges
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	(23,867)	(25,851)
Affiliated notes
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	635	829
Other investments
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	$ 1,075	$ 86