Debt - Schedule of Available Capacity Under Credit Agreement (Detail) - USD ($)		Mar. 31, 2018	Dec. 31, 2017
Letter of Credit Facility [Member]
Debt Instrument [Line Items]
Available capacity under current credit agreement	[1]	$ 510,000,000
Letters of credit utilized	[1]	390,000,000	$ 407,000,000
Revolving Credit Facility/Letters of Credit [Member]
Debt Instrument [Line Items]
Available capacity under current credit agreement		$ 300,000,000

[1] Includes $19 million of financial letters of credit as of March 31, 2018 and as of December 31, 2017.