Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Examination [Line Items]
Tax act provisional net tax benefit						$ 14
Effective tax rate	33.80%	5.30%		13.70%	58.60%
IRS
Income Tax Examination [Line Items]
Possible increase in U.S. taxable income			$ 105
Possible additional federal tax expense			$ 37