Fair Value Measurements on a Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Assets and liabilities measured at fair value on a recurring basis
Derivative Assets	$ 6,286		$ 8,107
Derivative Liabilities	17,100		25,042
Fair Value, Inputs, Level 2
Assets and liabilities measured at fair value on a recurring basis
Derivative Asset, Gross	28,273		28,291
Derivative Liability, Gross	39,087		45,226
Counterparty netting under agreements governing derivatives
Assets and liabilities measured at fair value on a recurring basis
Derivative Asset, Netting	(21,987)	[1]	(20,184)	[1]
Derivative Liability, Netting	$ (21,987)	[1]	$ (20,184)	[1]

[1] 1) Represents counterparty netting under agreements governing such derivatives.