Investments (Schedule of Net Realized and Unrealized Gains on Investments and Net Other-Than-Temporary Impairments) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Gain (Loss) on Investments [Line Items]
Net realized and unrealized gains on investments	$ 42,052	$ 59,870	$ 143,538	$ 191,295
Fixed maturity investments
Gain (Loss) on Investments [Line Items]
Gross realized gains	16,343	20,383	43,053	60,794
Gross realized losses	(6,126)	(3,363)	(29,902)	(25,832)
Net realized gains on fixed maturity investments	10,217	17,020	13,151	34,962
Net unrealized gains (losses) on investments trading	5,545	(4,235)	48,940	125,501
Derivatives
Gain (Loss) on Investments [Line Items]
Net realized and unrealized (losses) gains on investments-related derivatives	(4,020)	1,727	(4,344)	(26,873)
Equity investments
Gain (Loss) on Investments [Line Items]
Net realized gains on fixed maturity investments	13,675	127	49,736	14,038
Net unrealized gains (losses) on investments trading	$ 16,635	$ 45,231	$ 36,055	$ 43,667