Segment Information - Additional Information (Detail) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | Segment									4
Total revenues	$ 508,748	$ 530,850	$ 473,187	$ 501,461	$ 474,316	$ 475,646	$ 466,305	$ 465,080	$ 2,014,246	$ 1,881,347	$ 1,766,629
London, Bermuda and Cayman Islands
Segment Reporting Information [Line Items]
Total revenues									$ 15,200	$ 15,900	$ 14,500