Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Provision for income taxes	$ 272	$ 365	$ 529	$ 654
Effective tax rates	24.90%	35.80%	21.20%	33.20%
Provision for tax benefits related to settlement of employee awards	$ 4	$ 9	$ 96	$ 76
Provision for income tax benefit excluding gains on sale	122		$ 245
Effective tax rate			21.00%		35.00%
Liability for unrecognized tax benefits	461		$ 461		$ 439
Unrecognized tax benefits, accrued interest	54		54		44
Unrecognized tax benefits that would impact effective tax rate	$ 159		$ 159		$ 145