Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Estimated annual effective tax rate (as a percent)	24.00%	32.00%
Tax benefit for settlement of stock-based compensation awards	$ 40	$ 17
Prior year tax adjustment expense (benefit)		$ 15
U.S. 2017 tax reform estimated impact			$ 2,371
Tax Year Prior Years
Income Tax Contingency
Income tax examination, proposed liability increase/(decrease)	$ 2,300