Income Taxes - Reconciliation of income tax rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation
Statutory federal income tax rate on income from operations	35.00%	35.00%	35.00%
Tax Legislation benefit	(12.70%)	0.00%	0.00%
Share-based payments	(1.60%)	0.00%	0.00%
Tax-exempt income	(0.80%)	(1.20%)	(1.00%)
Tax credits	(0.90%)	(1.20%)	(0.90%)
Non-deductible goodwill impairment	1.10%	0.00%	0.00%
Other	0.00%	(0.70%)	0.80%
Effective income tax rate (as a percent)	20.10%	31.90%	33.90%
Income tax expense (benefit)	$ 802	$ 877	$ 1,111
Accounting Standards Update 2017-09
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation
Income tax expense (benefit)	$ (63)
Accounting Standards Update 2014-01
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation
Income tax expense (benefit)		$ 45