Commitments and Contingencies (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Letters of Credit Outstanding, Amount	$ 8,100	$ 16,045		$ 5,500
Total operating leases rental expense	$ 16,837	$ 17,936	$ 16,992

Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

No definition available.

Name:	us-gaap_LeaseAndRentalExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration