Goodwill and Purchased Intangible Assets - Amorization Expense (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization expense	$ 20	$ 64
Amortization expense for the remainder of the year	21	21
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
2019	76	76
2020	58	58
2021	50	50
2022	45	45
Finite-Lived Intangible Assets, Amortization Expense, after Year Five	$ 43	$ 43