Fair Value Measurements - Foreign Currency Forward Contracts (Details) - Foreign currency forward contracts designated as hedges - Designated as Hedges - Cash Flow Hedging - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Notional amount of foreign currency derivatives	$ 521,000,000	$ 346,000,000
Maximum length of time over which our foreign currency forward contracts mature	12 months
Fair value of foreign currency forward contracts	$ (5,000,000)	22,000,000
Net realized but unrecognized losses with remaining maturities of 12 months or less	10,000,000
Net realized but unrecognized losses recorded within accumulated other comprehensive income (loss)	10,000,000
Ineffective portion relating to these hedges	$ 0	$ 0