Segment Information - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Segment	Jun. 30, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | Segment			4
Total revenues	$ 473,187	$ 466,305	$ 974,648	$ 931,385
Document Period End Date			Jun. 30, 2018
London, Bermuda and Cayman Islands
Segment Reporting Information [Line Items]
Total revenues	$ 4,200	$ 4,200	$ 7,500	$ 7,200