Segment Information - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Segment	Sep. 30, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | Segment			4
Total revenues	$ 530,850	$ 475,646	$ 1,505,498	$ 1,407,031
Document Period End Date			Sep. 30, 2018
London, Bermuda and Cayman Islands
Segment Reporting Information [Line Items]
Total revenues	$ 3,700	$ 3,900	$ 11,300	$ 11,100