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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	$ 13,244	$ 12,233	$ 40,979	$ 36,763
Operating Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	699	598	2,056	1,647
Brokered Natural Gas and Marketing Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	531	452	1,310	1,313
Exploration Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	983	1,126	3,013	3,048
General and Administrative Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	$ 11,031	$ 10,057	$ 34,600	$ 30,755