Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Assets
Total assets	$ 1.5	$ 330.4
Liabilities
Total liabilities	1.0	1.5
Foreign Exchange Contract
Assets
Total assets	1.5	1.7
Liabilities
Total liabilities	1.0	1.5
Available-for-sale Securities
Assets
Total assets		328.7
Significant Other Observable Inputs (Level 2)
Assets
Total assets	1.5	330.4
Liabilities
Total liabilities	1.0	1.5
Significant Other Observable Inputs (Level 2) | Foreign Exchange Contract
Assets
Total assets	1.5	1.7
Liabilities
Total liabilities	$ 1.0	1.5
Significant Other Observable Inputs (Level 2) | Available-for-sale Securities
Assets
Total assets		$ 328.7