Fair Value Measurement - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets
Total assets		$ 330.4
Liabilities
Total liabilities		1.5
Available-for-sale Securities
Assets
Total assets		328.7
Foreign Currency Contracts
Assets
Total assets	$ 2.2	1.7
Liabilities
Total liabilities		1.5
Significant Other Observable Inputs (Level 2)
Assets
Total assets		330.4
Liabilities
Total liabilities		1.5
Significant Other Observable Inputs (Level 2) | Available-for-sale Securities
Assets
Total assets		328.7
Significant Other Observable Inputs (Level 2) | Foreign Currency Contracts
Assets
Total assets	$ 2.2	1.7
Liabilities
Total liabilities		$ 1.5