Fair Value Measurements (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2015 USD ($) Impaired_Asset_And_Liability	Jun. 30, 2014 Impaired_Asset_And_Liability	Dec. 31, 2014 USD ($)
Assets
Deferred compensation plan	$ 13,282		$ 13,115
Derivative contracts	76,178		137,603
Total assets	89,460		150,718
Liabilities
Deferred compensation plan	30,130		28,932
Total liabilities	$ 30,130		28,932
Number of non-financial assets and liabilities impaired | Impaired_Asset_And_Liability	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring basis
Assets
Deferred compensation plan	$ 13,282		13,115
Derivative contracts	0		0
Total assets	13,282		13,115
Liabilities
Deferred compensation plan	30,130		28,932
Total liabilities	30,130		28,932
Significant Other Observable Inputs (Level 2) | Recurring basis
Assets
Deferred compensation plan	0		0
Derivative contracts	28,180		51,645
Total assets	28,180		51,645
Liabilities
Deferred compensation plan	0		0
Total liabilities	0		0
Significant Unobservable Inputs (Level 3) | Recurring basis
Assets
Deferred compensation plan	0		0
Derivative contracts	47,998		85,958
Total assets	47,998		85,958
Liabilities
Deferred compensation plan	0		0
Total liabilities	$ 0		$ 0