Background (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Provisional income tax expense (benefit)				$ 1,100
Purchase accounting adjustment		$ 76
Reduction to provisional benefit		208
Proceeds from income tax refunds	$ 314			392
Cumulative effect of adoption of ASU				(407)
Reclassification from AOCI to retained earnings, tax effect		$ 407
Consumer Customers
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Amortization period			30 months
Minimum | Business Customer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Amortization period			12 months
Maximum | Business Customer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Amortization period			60 months
RETAINED EARNINGS | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of ASU				297	$ 0
Cumulative net effect of adoption, tax				$ (101)