Background (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended	12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Provisional income tax expense (benefit)					$ (1,100)
Purchase accounting adjustment				$ 83
Reduction to provisional benefit				$ 208
Proceeds from income tax refunds	$ 392	$ 314
Cumulative effect of adoption of ASU					(407)
Contract term				30 months
Reclassification from AOCI to retained earnings, tax effect			$ 407
Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract term				1 year
Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract term				7 years
Business Customer | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Length of customer life				12 months
Business Customer | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Length of customer life				60 months
Consumer Customers | Weighted Average
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Length of customer life				30 months
Accounting Standards Update 2014-09 | RETAINED EARNINGS
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of ASU		49			346	$ 0	$ 0
Cumulative net effect of adoption, tax		$ 17			$ 117