Customer Deposits - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Customer Deposits Disclosure [Abstract]
Customer deposits	$ 942.1	$ 853.9
Increase in customer deposits	$ 58.5

The increase (decrease) during the period in the amount of customer money held in customer accounts, including security deposits, collateral for a current or future transactions, initial payment of the cost of acquisition or for the right to enter into a contract or agreement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

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