Summary of Significant Accounting Policies - Recently Adopted Accounting Pronouncements (Details) - USD ($)	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Sep. 30, 2017	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncement, Early Adoption [Line Items]
Income tax benefit				$ (71,000,000)	$ (8,000,000)	$ 22,000,000
Cash from (used in) financing activities				(517,000,000)	(384,000,000)	(161,000,000)
Payments for debt issuance costs and discounts				(5,000,000)	(5,000,000)	0
Net cash from operating activities				478,000,000	380,000,000	122,000,000
Tax benefit related to the adoption of ASU 2016-09				7,000,000
Accounting Standards Update 2016-09, Excess Tax Benefit Component
New Accounting Pronouncement, Early Adoption [Line Items]
Cash from (used in) financing activities				(7,000,000)	(3,000,000)	(12,000,000)
Net cash from operating activities				7,000,000	$ 3,000,000	$ 12,000,000
Accounting Standards Update 2014-09 | Scenario, Forecast | Minimum
New Accounting Pronouncement, Early Adoption [Line Items]
Reduction to retained earnings	$ 17,000,000
Accounting Standards Update 2014-09 | Scenario, Forecast | Maximum
New Accounting Pronouncement, Early Adoption [Line Items]
Reduction to retained earnings	$ 20,000,000
New Accounting Pronouncement, Early Adoption, Effect | ASU 2016-16
New Accounting Pronouncement, Early Adoption [Line Items]
Reduction to retained earnings				9,000,000
Benefit related to intercompany transfer of intellectual property			$ 12,000,000	12,000,000
Income tax benefit				$ 0
New Accounting Pronouncement, Early Adoption, Effect | Accounting Standards Update 2016-15
New Accounting Pronouncement, Early Adoption [Line Items]
Cash from (used in) financing activities		$ 4,000,000
Payments for debt issuance costs and discounts		$ 5,000,000