New Accounting Standards (Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Assets
Other assets	$ 2,038	$ 2,116	$ 1,949
Equity
Retained earnings	15,222	14,608	14,408
Accumulated other comprehensive income (loss)	(260)	(185)	$ (152)	$ (132)	$ (163)
Capitalized contract cost	230	219
Accounting Standards Update 2014-09 and 2018-02 [Member]
Equity
Capitalized contract cost		219
Deferred tax liability		52
ASU 2014-09 [Member]
Equity
Capitalized contract cost	230
Deferred tax liability	$ 55	52
ASU 2014-09 [Member] | Adjustments [Member]
Assets
Other assets		167
Equity
Retained earnings		167
Accumulated other comprehensive income (loss)		0
ASU 2018-02 [Member] | Adjustments [Member]
Assets
Other assets		0
Equity
Retained earnings		33
Accumulated other comprehensive income (loss)		$ (33)