Derivatives - Fair Value of Derivative Instruments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Notional Value of Matured Swaps	$ 110.6
Derivative, Notional Amount		$ 1,048.2
Interest Rate		1.83%
Derivative, Fair Value, Net		$ 17.5	$ 9.7
Weighted average interest rate matured swaps	1.11%
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		$ 1.0	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		$ 2.7	1.9
Interest Rate Swaps 2021 Maturity [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount		$ 398.2
Interest Rate		1.57%
Derivative, Fair Value, Net		$ 12.6	$ 7.0
Interest Rate Swaps 2023 Maturity [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount		$ 50.0
Interest Rate		2.62%
Derivative, Fair Value, Net		$ 0.3
Interest Rate Forward Starting Swaps 2023 Maturity [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount		$ 250.0
Interest Rate		2.71%
Derivative, Fair Value, Net		$ 0.9