Fair Value Measurements - Changes in Level 3 Assets (Detail) - Auction Rate - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 4.7	$ 17.1
Total Gains (Losses):
Included in Other Comprehensive Income	0.2	(0.7)
Purchases, Issues, Sales, and Settlements
Sales	0.0	(10.1)
Settlements	(0.6)	(1.6)
Balance at end of period	$ 4.3	$ 4.7