Investments (Schedule of Net Realized and Unrealized Gains on Investments and Net Other-Than-Temporary Impairments) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Net realized and unrealized gains (losses) on investments	$ (7,716)	$ 42,052	$ 58,113	$ 43,373	$ (49,967)	$ 59,870	$ 69,772	$ 61,653	$ 135,822	$ 141,328	$ (68,918)
Fixed maturity investments
Gain (Loss) on Investments [Line Items]
Gross realized gains									49,121	72,739	50,488
Gross realized losses									(38,832)	(38,315)	(53,630)
Net realized gains (losses) on fixed maturity investments									10,289	34,424	(3,142)
Net unrealized gains (losses) on fixed maturity and equity investments trading									8,479	26,954	(64,908)
Net realized gains on equity investments trading									10,289	34,424	(3,142)
Investments-related derivatives
Gain (Loss) on Investments [Line Items]
Net realized and unrealized (losses) gains on investments-related derivatives									(2,490)	(15,414)	5,443
Equity investments
Gain (Loss) on Investments [Line Items]
Net realized gains (losses) on fixed maturity investments									80,027	14,190	16,348
Net unrealized gains (losses) on fixed maturity and equity investments trading									39,517	81,174	(22,659)
Net realized gains on equity investments trading									$ 80,027	$ 14,190	$ 16,348