Exit and Termination Costs - Exit and Termination Costs Included in Accrued Liabilities in Consolidated Balance Sheet - (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Exit costs included in accrued liabilities in consolidated balance sheet
Accrued severance costs		$ 0	$ (148)	$ 509	$ 1,447
Exit Costs
Exit costs included in accrued liabilities in consolidated balance sheet
Beginning balance	[1]			493,543
Accrued severance costs	[1]			0
Accretion of discount	[1]			25,424
Divestiture contract obligations - changes in estimate	[1]			(28,165)
Payments	[1]			(46,321)
Ending balance	[1]	444,481		444,481
Termination Costs
Exit costs included in accrued liabilities in consolidated balance sheet
Beginning balance				1,454
Accrued severance costs				509
Accretion of discount				0
Divestiture contract obligations - changes in estimate	[1]			0
Payments	[1]			(1,594)
Ending balance		$ 369		$ 369

[1] Includes the divestiture contract obligation and the transportation contract capacity release obligation.