Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Mar. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash provided by (used in) financing activities	$ 14,685	$ (1,000)	$ 12,507	$ (3,148)	$ 11,916	$ (4,746)
Net cash provided by (used in) operating activities	3,851	4,659	6,090	6,284	17,077	15,004
Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash provided by (used in) financing activities		(173)		(493)		(401)
Net cash provided by (used in) operating activities		$ 173		$ 493		$ 401
Accounting Standards Update 2016-09 | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative impact of adoption							$ 687
Adoption of ASU 2016-16
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets	$ 400		$ 400		$ 400