Fair Value (Change in Carrying Value of Level 3 Financial Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Fair value measurement Level 3 reconciliation [Abstract]
Balance as of beginning of period	$ 50,854	$ 58,926	$ 53,006	$ 58,926	$ 52,081
Transfer out	0	$ (26)	(1,607)	(26)	(1,607)
Purchases	0		0	198	2,821
Sales	0		0	(18,042)	(1,121)
Realized gains (losses), net	0		0	4,920	721
Unrealized (losses) gains, net	(1,268)		2,250	3,610	754
Balance as of end of period	$ 49,586		$ 53,649	$ 49,586	$ 53,649