Stock-Based Compensation Plans - Allocation of Stock-Based Compensation by Functional Category (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	$ 13,965	$ 13,244	$ 53,087	$ 40,979
Operating Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	720	699	3,509	2,056
Brokered Natural Gas And Marketing Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	656	531	2,314	1,310
Exploration Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	1,033	983	3,408	3,013
General and Administrative Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	$ 11,556	$ 11,031	$ 43,856	$ 34,600