Leases - Additional Information (Detail) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2018
Operating Leases, Rent Expense	$ 892,000	$ 1,776,000,000
Rental Income, Nonoperating	$ 10,000	$ 21,000,000

Income earned by providing the use of assets to an outside party in exchange for a payment or series of payments that is nonoperating in nature.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7)
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

Name:	us-gaap_RentalIncomeNonoperating
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration