Segment Information - Additional Information (Detail) $ in Thousands	3 Months Ended
Mar. 31, 2018 USD ($) Segment	Mar. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments \| Segment	4
Total revenues	$ 501,461	$ 465,080
London, Bermuda and Cayman Islands
Segment Reporting Information [Line Items]
Total revenues	$ 3,300	$ 3,000

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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