Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Loss from reclassification of contingent liability	$ 6,000,000
Transfers between fair value hierarchies	0	$ 0
Shopmium
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent consideration payout	$ 0
Contingent consideration milestones expiration period	Mar. 31, 2018
1.75% Convertible Senior Notes Due 2022 | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value of convertible senior notes	$ 207,800,000	$ 196,300,000
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed net asset value	$ 1