Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Assets
Total assets	$ 120.6	$ 330.4
Liabilities
Total liabilities	0.4	1.5
Foreign Exchange Forward Contracts
Assets
Total assets	0.2	1.7
Liabilities
Total liabilities	0.4	1.5
Significant Other Observable Inputs (Level 2)
Assets
Total assets	120.6	330.4
Liabilities
Total liabilities	0.4	1.5
Significant Other Observable Inputs (Level 2) | Foreign Exchange Forward Contracts
Assets
Total assets	0.2	1.7
Liabilities
Total liabilities	0.4	1.5
Available-for-Sale Securities
Assets
Total assets	120.4	328.7
Available-for-Sale Securities | Significant Other Observable Inputs (Level 2)
Assets
Total assets	$ 120.4	$ 328.7