Stock-Based Compensation Plans - Allocation of Stock-Based Compensation by Functional Category (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended			12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016		Jun. 30, 2015	Dec. 31, 2015	[1]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 16,888	$ 18,411	$ 29,795		$ 32,902
Direct Operating Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	696	654	1,284		1,540
Brokered Natural Gas and Marketing Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	378	619	894		1,125
Exploration Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	371	751	1,061	[1]	1,483	$ 2,985
General and Administrative Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	15,443	15,953	26,556		27,033
Termination Costs
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 0	$ 434	$ 0		$ 1,721

[1] Includes costs incurred whether capitalized or expensed.