Segment Information Geographic Area (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	$ 1,782	$ 1,663	$ 1,593	$ 1,478	$ 1,802	$ 1,677	$ 1,620	$ 1,444	$ 6,516	$ 6,543	$ 6,373
UNITED STATES
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									$ 3,224	$ 3,106	$ 2,909
Percentage of Revenues by Geographic Area									50.00%	47.00%	46.00%
Americas (Excluding United States) [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									$ 585	$ 637	$ 590
Percentage of Revenues by Geographic Area									9.00%	10.00%	9.00%
EMEA
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									$ 1,786	$ 1,896	$ 1,964
Percentage of Revenues by Geographic Area									27.00%	29.00%	31.00%
APJ [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									$ 921	$ 904	$ 910
Percentage of Revenues by Geographic Area									14.00%	14.00%	14.00%