Fair Value Measurements - Carrying Value and Fair Value of Debt (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Carrying Value | Revolving Credit Facility [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value	$ 615.0	$ 540.0
Carrying Value | Senior Unsecured Notes [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value	892.6	891.1
Fair Value | Revolving Credit Facility [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value	615.0	540.0
Fair Value | Senior Unsecured Notes [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value	$ 926.3	$ 919.2