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Credit Agreement - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 11, 2015
Line Of Credit Facility [Line Items]      
Cash collateral percentage 100.00%    
Credit Agreement [Member]      
Line Of Credit Facility [Line Items]      
Credit facility borrowings   $ 38,000,000  
Credit Agreement [Member] | Revolving Credit Facility [Member] | BMO Harris Bank N.A. (“BMO Harris”) [Member]      
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