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Credit Agreement - Additional Information (Detail) - Credit Agreement [Member] - Revolving Credit Facility [Member] - BMO Harris Bank N.A. (“BMO Harris”) [Member] - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 11, 2015
Line Of Credit Facility [Line Items]    
Revolving credit facility   $ 95,000,000
Revolving credit facility maturity date Dec. 31, 2018  
Number of days for pre-payment notice 3 days  
Number of days for termination notice 5 days  
Minimum cash under covenant term $ 110,000,000  
Cash collateral percentage 100.00%  
Credit facility borrowings $ 0