Derivatives - Fair Value of Derivative Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount	$ 748.6
Interest Rate	1.48%
Derivative, Fair Value, Net	$ 14.6	$ 9.7
Interest Rate Swaps 2019 Maturity [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount	$ 250.0
Interest Rate	1.44%
Derivative, Fair Value, Net	$ 1.2	0.8
Interest Rate Swaps 2020 Maturity [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount	$ 100.0
Interest Rate	1.23%
Derivative, Fair Value, Net	$ 2.5	1.9
Interest Rate Swaps 2021 Maturity [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount	$ 398.6
Interest Rate	1.57%
Derivative, Fair Value, Net	$ 10.9	$ 7.0