Fair Value Measurements (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 Impaired_Asset_And_Liabilty	Dec. 31, 2013 Impaired_Asset_And_Liabilty	Dec. 31, 2012 Impaired_Asset_And_Liabilty
Assets
Deferred compensation plan	$ 13,115,000	$ 12,507,000
Derivative contracts	137,603,000	13,792,000
Total assets	150,718,000	26,299,000
Liabilities
Deferred compensation plan	28,932,000	33,211,000
Derivative contracts	0	24,685,000
Total liabilities	28,932,000	57,896,000
Number of non-financial assets and liabilities impaired	0	0	0
Recurring Basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Deferred compensation plan	13,115,000	12,507,000
Derivative contracts	0	0
Total assets	13,115,000	12,507,000
Liabilities
Deferred compensation plan	28,932,000	33,211,000
Derivative contracts	0	0
Total liabilities	28,932,000	33,211,000
Recurring Basis | Significant Other Observable Inputs (Level 2)
Assets
Deferred compensation plan	0	0
Derivative contracts	51,645,000	0
Total assets	51,645,000	0
Liabilities
Deferred compensation plan	0	0
Derivative contracts	0	6,983,000
Total liabilities	0	6,983,000
Recurring Basis | Significant Unobservable Inputs (Level 3)
Assets
Deferred compensation plan	0	0
Derivative contracts	85,958,000	13,792,000
Total assets	85,958,000	13,792,000
Liabilities
Deferred compensation plan	0	0
Derivative contracts	0	17,702,000
Total liabilities	$ 0	$ 17,702,000