Derivative Instruments and Hedging Activities - Schedule of Amounts Recorded On Balance Sheet Related To Cumulative Basis Adjustments For Fair Value Hedges (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Carrying Amount of Hedged Liabilites [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Long-term debt	$ 570.7	$ 572.8
Cumulative Amount of Fair Value Hedging Adjustment Included in Carrying Amount of Hedged Liabilities [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Long-term debt	$ 21.0	$ 23.1