Fair Value Measurements--Derivatives Offset in the Consolidated Balance Sheets (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Fair Value Disclosures [Abstract]
Derivative assets, gross	$ 13,113	$ 46,010
Derivative assets, amounts offset	(8,042)	(4,721)
Derivative assets, net of offsetting liabilities	5,071	41,289
Credit facility balance available to offset net derivative assets	(4,211)	(9,180)
Derivative assets, net of offsetting amounts outstanding under credit facility, not offset	860	32,109
Derivative liabilities, gross	(12,792)	(5,605)
Derivative liabilities, amounts offset	8,042	4,721
Derivative liabilities, net of offsetting assets	(4,750)	(884)
Derivative assets (liabilities), net	321	40,405
Derivative assets (liabilities), net of offsetting amounts outstanding under credit facility, not offset	$ (3,890)	$ 31,225