Accounting Policies - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Accounting Policies [Line Items]
Anti-dilutive shares excluded from computation of income (loss) per share	0	9,600,000	0	9,600,000
Class A Common Stock [Member] | Put Option [Member]
Accounting Policies [Line Items]
Shares issued					200,000
Put right, exercise price					$ 12.50
Put right exercise period from date of purchase					5 years
Ministry of the Economy, Finance and Industry, France [Member] | Research and Development Tax Credit Carryforward [Member]
Accounting Policies [Line Items]
Tax credit carryforward, period			3 years
Tax credit carryforward	$ 10.4		$ 10.4
Income tax receivable [Member] | Ministry of the Economy, Finance and Industry, France [Member] | Research and Development Tax Credit Carryforward [Member]
Accounting Policies [Line Items]
Tax credit carryforward	2.3		2.3
Other long-term assets [Member] | Ministry of the Economy, Finance and Industry, France [Member] | Research and Development Tax Credit Carryforward [Member]
Accounting Policies [Line Items]
Tax credit carryforward	8.1		8.1
Prepaid Expenses and Other Current Assets [Member]
Accounting Policies [Line Items]
Asset held for sale of building and related assets	$ 2.1		$ 2.1