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Commitments and Contingencies - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Commitments And Contingencies Disclosure [Line Items]          
Capital lease assets, gross $ 65,562   $ 65,562   $ 65,238
Assets fully depreciated     19,395   20,499
Capital leased assets, net book value 24,043   24,043   28,754
Accumulated depreciation 41,519   41,519   $ 36,484
Actual purchases made under contract $ 18,520 $ 16,519 $ 49,656 $ 37,359  
Contract One [Member]          
Commitments And Contingencies Disclosure [Line Items]          
Extended term of contract     Dec. 31, 2015    
Contract Two [Member]          
Commitments And Contingencies Disclosure [Line Items]          
Extended term of contract     Aug. 31, 2017    
Minimum [Member]          
Commitments And Contingencies Disclosure [Line Items]          
Estimated life of capital lease     4 years    
Noncancellable operating leases, renewal period     1 year    
Maximum [Member]          
Commitments And Contingencies Disclosure [Line Items]          
Estimated life of capital lease     6 years    
Noncancellable operating leases, renewal period     5 years