Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss Contingencies [Line Items]
Payments for Legal Settlements	$ 4,200
Standard and Extended Product Warranty Accrual, Increase (Decrease) for Preexisting Warranties	$ (13,887)	$ (10,855)	$ (982)
Standard Product Warranty Accrual, Increase for Warranties Issued		(1,900)
ARPA Trial [Member]
Loss Contingencies [Line Items]
Standard and Extended Product Warranty Accrual, Increase (Decrease) for Preexisting Warranties		$ 2,300