Fair Value (Change in Carrying Value of Level 3 Financial Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Fair value measurement Level 3 reconciliation [Abstract]
Balance as of beginning of period	$ 58,841	$ 58,926	$ 50,744	$ 58,926	$ 52,081
Transfers out	0	26	0	(26)	0
Purchases	76		2,320	198	2,821
Sales	(18,042)		0	(18,042)	(1,121)
Realized (losses) gains, net	10,114		(400)	4,920	721
Unrealized gains (losses), net	(135)		342	4,878	(1,496)
Balance as of end of period	$ 50,854	$ 58,841	$ 53,006	$ 50,854	$ 53,006