Income Before Income Taxes from its Non-U.S. Subsidiaries and U.S. Subsidiaries (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
Revenues:
Gross premiums written									$ 547,897	$ 516,334	$ 565,845
Net premiums written									472,547	450,180	470,940
Net premiums earned	$ 125,328	$ 120,528	$ 113,917	$ 108,002	$ 109,216	$ 108,619	$ 107,073	$ 113,126	467,775	438,034	468,465
Net investment income	12,234	11,750	10,954	11,404	11,705	10,134	8,840	8,644	46,342	39,323	33,983
Net realized investment gains (losses)	(24,740)	5,319	2,830	(316)	2,426	(963)	(662)	775	(16,907)	1,576	21,721
Other income (loss)									1,728	6,582	10,345
Total revenues									498,938	485,515	534,514
Losses and Expenses:
Net losses and loss adjustment expenses	139,199	80,493	58,861	56,072	66,556	82,395	57,700	62,561	334,625	269,212	264,003
Acquisition costs and other underwriting expenses	49,582	48,680	47,513	45,003	48,723	45,002	43,457	46,551	190,778	183,733	196,650
Corporate and other operating expenses									29,766	25,714	17,338
Interest expense									19,694	16,906	8,905
Income (loss) before income taxes	$ (86,602)	$ 436	$ 5,793	$ 4,448	$ (10,304)	$ (16,779)	$ 7,753	$ 9,280	(75,925)	(10,050)	47,618
Non-U.S. Subsidiaries
Revenues:
Gross premiums written									48,050	212,386	201,726
Net premiums written									48,041	212,432	201,690
Net premiums earned									135,826	201,165	212,325
Net investment income									49,699	56,890	48,807
Net realized investment gains (losses)									(669)	(641)	(89)
Other income (loss)									(210)	216	(224)
Total revenues									184,646	257,630	260,819
Losses and Expenses:
Net losses and loss adjustment expenses									91,178	94,903	95,812
Acquisition costs and other underwriting expenses									57,487	89,153	94,749
Corporate and other operating expenses									12,234	17,399	9,035
Interest expense									7,108	16,740	8,312
Income (loss) before income taxes									16,639	39,435	52,911
U.S. Subsidiaries
Revenues:
Gross premiums written									499,847	462,453	506,061
Net premiums written									424,506	237,748	269,250
Net premiums earned									331,949	236,869	256,140
Net investment income									27,294	24,609	19,341
Net realized investment gains (losses)									(16,238)	2,217	21,810
Other income (loss)									1,938	6,366	10,569
Total revenues									344,943	270,061	307,860
Losses and Expenses:
Net losses and loss adjustment expenses									243,447	174,309	168,191
Acquisition costs and other underwriting expenses									133,291	94,580	101,901
Corporate and other operating expenses									17,532	8,315	8,303
Interest expense									43,237	42,342	34,758
Income (loss) before income taxes									(92,564)	(49,485)	(5,293)
Eliminations
Revenues:
Gross premiums written									0	(158,505)	(141,942)
Net premiums written									0	0	0
Net premiums earned									0	0	0
Net investment income									(30,651)	(42,176)	(34,165)
Net realized investment gains (losses)									0	0	0
Other income (loss)									0	0	0
Total revenues									(30,651)	(42,176)	(34,165)
Losses and Expenses:
Net losses and loss adjustment expenses									0	0	0
Acquisition costs and other underwriting expenses									0	0	0
Corporate and other operating expenses									0	0	0
Interest expense									(30,651)	(42,176)	(34,165)
Income (loss) before income taxes									$ 0	$ 0	$ 0