Leases, User Charges and Commitments - Additional Information (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($)
Lease And Rental Expense [Line Items]
Right-of-use asset	$ 39.2
Lease liability	40.2
Expected annual subleases income	0.6
ASU No. 2016-02 [Member]
Lease And Rental Expense [Line Items]
Estimated ROU assets and lease liabilities	$ 17.6