Contingent Liabilities and Commitments - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2015
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 97	$ 72	$ 39
Service commitment period	7 years
Total rent expenses		25	$ 19
Powder River [Member]
Loss Contingencies [Line Items]
Asset Retirement Obligation				$ 46
Surface Use Agreement Payments				$ 6
Royalty Litigation
Loss Contingencies [Line Items]
Loss contingencies associated with royalty litigation	$ 10	$ 11