Income Taxes - Reconciliation of Effective Income Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of the applicable statutory U.S. income tax rate to the effective tax rate
Statutory U.S. federal tax rate	21.00%	35.00%	35.00%
Increase/(decrease) in tax rate resulting from:
U.S. state and local income taxes, net of U.S. federal income tax benefit	4.00%	2.20%	2.40%
Tax-exempt income	(1.50%)	(3.30%)	(3.10%)
Non-U.S. subsidiary earnings	0.60%	(3.10%)	(1.70%)
Business tax credits	(3.50%)	(4.20%)	(3.90%)
Impact of the TCJA	(0.70%)	5.40%	0.00%
Other, net	0.40%	(0.10%)	(0.30%)
Effective tax rate	20.30%	31.90%	28.40%
Reconciliation of effective income tax rate, supplemental information [Abstract]
Net tax benefit recorded	$ 302
Income tax expense, estimated impact of TCJA enactment		$ 1,900
Estimated income tax expense from transition tax on accumulated earnings of controlled foreign corporations	3,700
Estimated income tax benefit as a result of TCJA remeasurement	$ 2,100