Segment Information - Revenue from External Customers by Products and Services (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
Revenue from External Customer [Line Items]
Product revenue									$ 2,579	$ 2,737	$ 2,711
Professional and installation services revenue									1,055	1,011	944
Recurring revenue, including maintenance and cloud revenue									2,882	2,795	2,718
Total revenue	$ 1,782	$ 1,663	$ 1,593	$ 1,478	$ 1,802	$ 1,677	$ 1,620	$ 1,444	$ 6,516	$ 6,543	$ 6,373