Summary of Gross Written Premiums Allocated Based on Location of Insured's Head Office (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 464,558	$ 455,943	$ 497,236	$ 495,224	$ 408,743	$ 402,038	$ 452,179	$ 450,305	$ 1,912,961	$ 1,713,265	$ 1,568,911
United States
Segment Reporting Information [Line Items]
Gross Written Premiums									1,373,346	1,203,695	1,101,337
Worldwide (Excluding the United States)
Segment Reporting Information [Line Items]
Gross Written Premiums									$ 539,615	$ 509,570	$ 467,574