Fair value measurements Fair Value Measurements--Derivatives Offset in the Consolidated Balance Sheets (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Fair Value Disclosures [Abstract]
Derivative assets, gross	$ 1,564		$ 13,113
Derivative assets, amount offset	(1,261)		(4,558)
Derivative assets	303		8,555
Derivative assets, not offset	(303)	[1]	(3,484)	[1]
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	(65,236)		(12,792)
Derivative liabilities, amounts offset	1,261		4,558
Derivative liabilities	63,975		8,234
Derivative liabilities, not offset	303	[1]	3,484	[1]
Derivative liabilities, net of offsetting amounts	(63,672)		(4,750)
Derivative assets (liabilities), net	(63,672)		321
Derivative assets (liabilities), net of offsetting amounts outstanding under credit facility, not offset	$ (63,672)		$ (3,890)

[1] Since positive and negative positions with a counterparty are netted on the balance sheet only to the extent that they relate to the same current versus noncurrent classification, these represent remaining amounts that could have been offset under our master netting agreements.