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Supplementary Balance Sheet Information - Changes in Deferred Revenues (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Deferred Revenue Disclosure [Abstract]    
Balance at beginning of year $ 24,177 $ 22,221
Revenue recognized during the year (34,849) (35,901)
Deferred income acquired   321
Deferred income relating to new sales 42,756 37,536
Balance at end of year $ 32,084 $ 24,177