Investments (Schedule of Net Realized and Unrealized (Losses) Gains on Investments) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Gain (Loss) on Investments [Line Items]
Net realized and unrealized (losses) gains on investments	$ (82,144)	$ 43,373
Fixed maturity investments
Gain (Loss) on Investments [Line Items]
Gross realized gains	4,583	11,461
Gross realized losses	(25,853)	(16,533)
Net realized gains on fixed maturity investments	(21,270)	(5,072)
Net unrealized gains (losses) on investments trading	(55,372)	24,635
Derivatives
Gain (Loss) on Investments [Line Items]
Net realized and unrealized losses on investments-related derivatives	(4,364)	(56)
Equity investments
Gain (Loss) on Investments [Line Items]
Net realized gains on fixed maturity investments	234	20,915
Net unrealized gains (losses) on investments trading	(1,372)	2,951
Net realized and unrealized (losses) gains on equity investments trading	$ (1,138)	$ 23,866