Stock-Based Compensation Plans - Allocation of Stock-Based Compensation by Functional Category (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	$ 14,137,000	$ 12,286,000
Operating Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	852,000	661,000
Brokered Natural Gas And Marketing Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	528,000	249,000
Exploration Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	1,153,000	1,070,000
General and Administrative Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	$ 11,604,000	$ 10,306,000