Recent Accounting Pronouncements (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets		$ 14,660		$ 14,660			$ 14,874
Liabilities		(9,560)		(9,560)			$ (9,621)
Equipment sales	[1],[2]	2,200	$ 2,352	6,695	$ 7,297
ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets						$ 385
Assets, undiscounted						440
Liabilities						(385)
Liabilities, undiscounted						$ (440)
Retained Earnings | ASU 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting principle				127
AOCL | ASU 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting principle				(127)
Equipment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equipment sales		494	$ 511	1,446	$ 1,571
Equipment | ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equipment sales		$ 9		$ 20

[1] Certain prior year amounts have been revised to conform to the current year presentation. Refer to Note 1 - Basis of Presentation - Change in Presentation, for additional information.

[2] Geographic area data is based upon the location of the subsidiary reporting the revenue.