Fair Value Measurements - Assets and Liabilities on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets
Deferred compensation plan	$ 14,966	$ 12,587
Derivative instruments	2,239	2,991
Total assets	17,205	15,578
Liabilities
Deferred compensation plan	29,145	24,169
Derivative instruments	30,637	40,259
Total liabilities	59,782	64,428
Recurring Basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Deferred compensation plan	14,966	12,587
Derivative instruments	0	0
Total assets	14,966	12,587
Liabilities
Deferred compensation plan	29,145	24,169
Derivative instruments	0	0
Total liabilities	29,145	24,169
Recurring Basis | Significant Other Observable Inputs (Level 2)
Assets
Deferred compensation plan	0	0
Derivative instruments	0	0
Total assets	0	0
Liabilities
Deferred compensation plan	0	0
Derivative instruments	0	21,400
Total liabilities	0	21,400
Recurring Basis | Significant Unobservable Inputs (Level 3)
Assets
Deferred compensation plan	0	0
Derivative instruments	2,239	2,991
Total assets	2,239	2,991
Liabilities
Deferred compensation plan	0	0
Derivative instruments	30,637	18,859
Total liabilities	$ 30,637	$ 18,859