Commitments and Contingencies - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Commitments And Contingencies Disclosure [Line Items]
Capital lease assets, gross	$ 65,238	$ 54,004
Assets fully depreciated	20,499	22,160
Capital leased assets, net book value	28,754	23,623
Accumulated depreciation	36,484	30,382
Rent expense under operating leases	7,871	7,171	6,343
Actual purchases made under contract	53,084	25,884
Total settlement costs incurred for lawsuits		1,407
Other accrued expense current	$ 0	$ 1,200
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