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Leases, User Charges and Commitments - Additional Information (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2019
Apr. 01, 2019
Jan. 01, 2019
Lease And Rental Expense [Line Items]      
Expected annual subleases income $ 0.5    
ASU No. 2016-02 [Member]      
Lease And Rental Expense [Line Items]      
Right-of-use asset     $ 40.0
Lease liability     $ 41.4
ASU No. 2016-02 [Member] | Subsequent Event [Member]      
Lease And Rental Expense [Line Items]      
Estimated ROU assets and lease liabilities   $ 10.2