Fair Value of Financial Instruments - Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - Available-for-Sale Securities [Member] - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 3.5	$ 3.3
Total realized gains (losses) - included in earnings	0.0	0.0
Total unrealized gains (losses) - included in other comprehensive income	0.0	0.0
Purchases	0.1	0.2
Sales	0.0	0.0
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Ending Balance	$ 3.6	$ 3.5