Stock-Based Compensation Plans - Allocation of Stock-Based Compensation by Functional Category (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	$ 24,985	$ 15,449	$ 39,122	$ 27,735
Operating Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	1,937	696	2,789	1,357
Brokered Natural Gas And Marketing Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	1,130	530	1,658	779
Exploration Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	1,222	960	2,375	2,030
General and Administrative Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	$ 20,696	$ 13,263	$ 32,300	$ 23,569