Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013 item	Dec. 31, 2012 item	Dec. 31, 2011 item
Financial Assets and Liabilities Fair Value Measurement
Number of non-financial assets and liabilities impaired	0	0	0
Assets
Deferred compensation plan	$ 12,507	$ 10,608
Derivative contracts	13,792	54,454
Total assets	26,299	65,062
Liabilities
Deferred compensation plan	33,211	23,893
Derivative contracts	24,685	3,822
Total liabilities	57,896	27,715
Transfer of Assets from Level 1 to Level 2	0	0	0
Transfer of Liabilities from Level 1 to Level 2	0	0	0
Transfer of Assets from Level 2 to Level 1	0	0	0
Transfer of Liabilities from Level 2 to Level 1	0	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Deferred compensation plan	12,507	10,608
Total assets	12,507	10,608
Liabilities
Deferred compensation plan	33,211	23,893
Total liabilities	33,211	23,893
Significant Other Observable Inputs (Level 2)
Assets
Derivative contracts		9,473
Total assets		9,473
Liabilities
Derivative contracts	6,983
Total liabilities	6,983
Significant Unobservable Inputs (Level 3)
Assets
Derivative contracts	13,792	44,981
Total assets	13,792	44,981
Liabilities
Derivative contracts	17,702	3,822
Total liabilities	$ 17,702	$ 3,822