Fair value measurements Fair Value Measurements--Derivatives Offset in the Consolidated Balance Sheets (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Fair Value Disclosures [Abstract]
Derivative assets, gross	$ 3,100	$ 13,113
Derivative assets, amounts offset	(3,100)	(8,042)
Derivative assets, net of offsetting liabilities	0	5,071
Credit facility balance available to offset net derivative assets	0	(4,211)
Derivative assets, net of offsetting amounts outstanding under credit facility, not offset	0	860
Derivative liabilities, gross	(23,223)	(12,792)
Derivative liabilities, amounts offset	3,100	8,042
Derivative liabilities, net of offsetting amounts	(20,123)	(4,750)
Derivative assets (liabilities), net	(20,123)	321
Derivative assets (liabilities), net of offsetting amounts outstanding under credit facility, not offset	$ (20,123)	$ (3,890)