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Leases - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Lessee Lease Description [Line Items]    
Operating lease, existence of option to extend true  
Operating lease, options to extend Some of these leases have options to extend which, if exercised, would extend the maximum term to approximately 24 years.  
Operating lease, existence of option to terminate true  
Operating lease, options to terminate Some of the leases also provide for early termination, the exercise of which would shorten the term of those leases by up to 5 years.  
Operating lease, expense $ 7.3  
Rent expense for office space   $ 6.3
Maximum [Member]    
Lessee Lease Description [Line Items]    
Operating lease, remaining lease terms 14 years  
Operating lease, extended term 24 years  
Operating lease, early termination term 8 years