Fair Value Measurements - Financial Assets and Liabilities, Recurring (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Assets
Deferred compensation plan	$ 12,586	$ 13,115
Derivative instruments	48,445	137,603
Total assets	61,031	150,718
Liabilities
Deferred compensation plan	24,263	28,932
Total liabilities	24,263	28,932
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring basis
Assets
Deferred compensation plan	12,586	13,115
Derivative instruments	0	0
Total assets	12,586	13,115
Liabilities
Deferred compensation plan	24,263
Total liabilities	24,263	28,932
Significant Other Observable Inputs (Level 2) | Recurring basis
Assets
Deferred compensation plan	0	0
Derivative instruments	16,152	51,645
Total assets	16,152	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 instruments	32,293	85,958
Total assets	32,293	85,958
Liabilities
Deferred compensation plan	0	0
Total liabilities	$ 0	$ 0