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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 408,743	$ 402,038	$ 452,179	$ 450,305	$ 367,539	$ 374,930	$ 412,565	$ 413,877	$ 1,713,265	$ 1,568,911	$ 1,453,502
United States
Segment Reporting Information [Line Items]
Gross Written Premiums									1,203,695	1,101,337	1,057,895
Worldwide (Excluding the United States)
Segment Reporting Information [Line Items]
Gross Written Premiums									$ 509,570	$ 467,574	$ 395,607