Recent Accounting Pronouncements (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets		$ 14,684		$ 14,684		$ 14,874
Liabilities		(9,478)		(9,478)		$ (9,621)
Equipment sales	[1],[2]	2,289	$ 2,510	4,495	$ 4,945
ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets		385		385
Assets, undiscounted		440		440
Liabilities		(385)		(385)
Liabilities, undiscounted		(440)		(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		$ 504	$ 561	952	$ 1,060
Equipment | ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equipment sales				$ 11

[1] Certain prior year amounts have been conformed to the current year presentation. Refer to Note 1 - Basis of Presentation for additional information.

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