Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating profit										$ 3,589	$ 3,837	$ 2,789
Net income	$ 323	$ 607	$ 524	$ 570	$ 606	$ 702	$ 600	$ 533		2,024	$ 2,441	$ 1,384
Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Excess tax benefits recognized										47
Accounting Standards Update 2017-07 | Scenario, Forecast
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating profit									$ 100
Net income									$ 0
Accounting Standards Update 2017-07 | Pro Forma
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating profit										120
Net income										$ 0