Other Assets - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Other Assets [Line Items]
Gross carrying amount of intangible assets	$ 52.8	$ 47.4
Amortization expense related to intangible assets	3.4	2.9	$ 1.5
Intangible lease value
Schedule of Other Assets [Line Items]
Gross carrying amount of intangible assets	$ 41.6	$ 41.6