Changes in Accounting Principles (Details) - USD ($) $ in Millions		3 Months Ended
	Jan. 01, 2019	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income taxes reclassified to retained earnings	$ 89			$ 89
Cumulative effect of accounting changes				(9)	$ 49
Income tax expense		$ 70	$ 132
Right-of-use assets	1,415	1,376
Lease liability	1,415	$ 1,333
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting changes				$ 81	$ 36
Retained Earnings | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting changes	9
Income tax expense	$ 3