Fair Value Measurements - Assets and Liabilities on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets
Deferred compensation plan	$ 12,921	$ 13,115
Derivative instruments	0	137,603
Total assets	12,921	150,718
Liabilities
Deferred compensation plan	22,371	28,932
Total liabilities	22,371	28,932
Recurring Basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Deferred compensation plan	12,921	13,115
Derivative instruments		0
Total assets	12,921	13,115
Liabilities
Deferred compensation plan	22,371	28,932
Total liabilities	22,371	28,932
Recurring Basis | Significant Other Observable Inputs (Level 2)
Assets
Deferred compensation plan	0	0
Derivative instruments		51,645
Total assets	0	51,645
Liabilities
Deferred compensation plan	0	0
Total liabilities	0	0
Recurring Basis | Significant Unobservable Inputs (Level 3)
Assets
Deferred compensation plan	0	0
Derivative instruments		85,958
Total assets	0	85,958
Liabilities
Deferred compensation plan	0	0
Total liabilities	$ 0	$ 0