Stock Compensation Plans (Schedule of Share-Based Compensation) (Details 1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expensed	$ 30,513	$ 33,003	$ 29,310
Stock-based compensation capitalized	9,019	9,088	8,587
Total cost of stock-based compensation arrangements	39,532	42,091	37,897
Income tax benefit recognized for stock-based compensation arrangements	11,595	12,541	11,284
General and administrative expenses [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expensed	27,789	30,429	26,463
Lease operating expenses [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expensed	$ 2,724	$ 2,574	$ 2,847