Segment Information - Additional Information (Detail) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | Segment									4
Total revenues	$ 474,316	$ 475,646	$ 466,305	$ 465,080	$ 433,664	$ 462,274	$ 446,518	$ 424,173	$ 1,881,347	$ 1,766,629	$ 1,660,509
London, Bermuda and Cayman Islands
Segment Reporting Information [Line Items]
Total revenues									$ 15,900	$ 14,500	$ 13,400