Restructuring Expenses - Additional Information (Detail) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)
Restructuring And Related Cost [Line Items]
Severance agreement description		The costs for severance agreements are being accrued ratably over the remaining service periods, which span intermittent periods from July 2017 through April 2018, and the liabilities are to be paid intermittently from November 2017 through May 2018.
Operations and Maintenance [Member]
Restructuring And Related Cost [Line Items]
Severance expenses	$ 2.1
Depreciation and Amortization [Member]
Restructuring And Related Cost [Line Items]
Accelerated amortization of leasehold improvements	0.5
Other Current Liabilities [Member]
Restructuring And Related Cost [Line Items]
Severance and lease restructuring charges reserve	$ 2.1	$ 2.1