Background (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle
Retained earnings	$ 1,336		$ 1,103
Reclassification from AOCI to retained earnings, tax effect	407
Provisional income tax expense (benefit)			(1,100)
Purchase accounting adjustment	64
Business Customer
New Accounting Pronouncements or Change in Accounting Principle
Deferred customer contract, amortization period		49 months
Consumer Customers
New Accounting Pronouncements or Change in Accounting Principle
Deferred customer contract, amortization period		30 months
Accounting Standards Update 2014-09 | Impact of 606
New Accounting Pronouncements or Change in Accounting Principle
Retained earnings	$ (293)		$ 297