Consolidated Balance Sheets - (Parenthetical) - USD ($) shares in Thousands, $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accounts receivable allowances	$ 52	$ 42
Property and equipment, accumulated depreciation	1,585	1,110
Identifiable intangible assets, accumulated amortization	$ 706	$ 560
Preferred stock, par value (in usd per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	10,000	10,000
Preferred stock, shares issued (in shares)	70	70
Preferred stock, shares outstanding (in shares)	70	70
Common stock, par value (in usd per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	300,000	300,000
Common stock, shares issued (in shares)	116,000	120,000
Common stock, shares outstanding (in shares)	116,000	120,000