Financial Data by Segment (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
Ceded written premiums										(441,876)	(441,935)	(382,687)
Net written premiums		346,119	352,412	379,292	393,262	304,869	296,016	333,282	337,163	$ 1,471,085	$ 1,271,330	$ 1,186,224
Retention Ratio										76.90%	74.20%	75.60%
Net Earned Premiums		362,543	347,038	331,015	322,627	305,099	301,355	293,835	286,131	$ 1,363,223	$ 1,186,420	$ 1,100,345
Net Losses and LAE		(270,401)	(240,900)	(196,333)	(186,145)	(183,384)	(276,171)	(177,110)	(169,600)	(893,779)	(806,265)	(665,448)
Commission Expenses		(54,800)	(57,999)	(53,193)	(54,152)	(43,205)	(45,509)	(48,173)	(47,844)	(220,144)	(184,731)	(165,045)
Other Operating Expenses		(62,532)	(67,584)	(68,182)	(62,926)	(68,153)	(45,773)	(60,766)	(58,538)	(261,224)	(233,230)	(234,096)
Other Underwriting Income (Expense)										2,311	68	136
Underwriting Profit (Loss)										(9,613)	(37,738)	35,892
Net Investment Income		29,159	25,651	24,601	23,702	22,982	22,598	22,265	21,448	103,113	89,293	79,451
Total Net Realized and Unrealized Losses		(34,015)	2,563	3,116	(2,012)					(30,348)	43,009	9,036
Interest Expense		(3,866)	(3,891)	(3,864)	(3,864)	(3,863)	(3,862)	(3,861)	(3,861)	(15,485)	(15,447)	(15,435)
Other Income										4,858	(4,311)	8,565
Merger Transaction Costs		(1,511)	(2,439)							(3,950)	0	0
Income Before Income Taxes		(32,489)	4,163	39,788	37,113	63,416	(44,843)	27,473	28,760	48,575	74,806	117,509
Income Tax Expense		(879)	462	(7,684)	(6,235)	(36,555)	16,864	(6,971)	(7,650)	(14,336)	(34,312)	(34,783)
Net Income		$ (33,368)	$ 4,625	$ 32,104	$ 30,878	$ 26,861	$ (27,979)	$ 20,502	$ 21,110	$ 34,239	$ 40,494	$ 82,726
Losses and LAE ratio										65.60%	68.00%	60.50%
Commission Expense Ratio										16.10%	15.60%	15.00%
Other Operating Expense Ratio	[1]									19.00%	19.60%	21.20%
Combined Ratio		106.60%	105.30%	96.00%	94.00%	96.60%	121.90%	97.30%	96.40%	100.70%	103.20%	96.70%
U.S.Insurance
Segment Reporting Information [Line Items]
Gross Written Premiums										$ 1,083,116	$ 988,293	$ 919,395
Ceded written premiums										(283,025)	(266,993)	(235,827)
Net written premiums										$ 800,091	$ 721,300	$ 683,568
Retention Ratio										73.90%	73.00%	74.30%
Net Earned Premiums										$ 737,646	$ 674,665	$ 629,308
Net Losses and LAE										(485,960)	(443,353)	(397,860)
Commission Expenses										(85,127)	(77,729)	(70,812)
Other Operating Expenses										(142,333)	(128,905)	(128,108)
Other Underwriting Income (Expense)										354	461	1,092
Underwriting Profit (Loss)										24,580	25,139	33,620
Income Before Income Taxes										$ 24,580	$ 25,139	$ 33,620
Losses and LAE ratio										65.90%	65.70%	63.20%
Commission Expense Ratio										11.50%	11.50%	11.30%
Other Operating Expense Ratio	[1]									19.30%	19.10%	20.20%
Combined Ratio										96.70%	96.30%	94.70%
International Insurance
Segment Reporting Information [Line Items]
Gross Written Premiums										$ 530,710	$ 501,130	$ 484,471
Ceded written premiums										(147,402)	(165,278)	(138,504)
Net written premiums										$ 383,308	$ 335,852	$ 345,967
Retention Ratio										72.20%	67.00%	71.40%
Net Earned Premiums										$ 380,503	$ 333,792	$ 307,416
Net Losses and LAE										(252,268)	(229,601)	(178,284)
Commission Expenses										(80,526)	(68,824)	(61,703)
Other Operating Expenses										(93,882)	(83,464)	(86,395)
Other Underwriting Income (Expense)										2,206	0	0
Underwriting Profit (Loss)										(43,967)	(48,097)	(18,966)
Income Before Income Taxes										$ (43,967)	$ (48,097)	$ (18,966)
Losses and LAE ratio										66.30%	68.80%	58.00%
Commission Expense Ratio										21.20%	20.60%	20.10%
Other Operating Expense Ratio	[1]									24.10%	25.00%	28.10%
Combined Ratio										111.60%	114.40%	106.20%
Global Reinsurance
Segment Reporting Information [Line Items]
Gross Written Premiums										$ 299,135	$ 223,842	$ 165,045
Ceded written premiums										(11,449)	(9,664)	(8,356)
Net written premiums										$ 287,686	$ 214,178	$ 156,689
Retention Ratio										96.20%	95.70%	94.90%
Net Earned Premiums										$ 245,074	$ 177,963	$ 163,621
Net Losses and LAE										(155,551)	(133,311)	(89,304)
Commission Expenses										(55,056)	(39,136)	(34,008)
Other Operating Expenses										(25,009)	(20,861)	(19,593)
Other Underwriting Income (Expense)										316	565	522
Underwriting Profit (Loss)										9,774	(14,780)	21,238
Income Before Income Taxes										$ 9,774	$ (14,780)	$ 21,238
Losses and LAE ratio										63.50%	74.90%	54.60%
Commission Expense Ratio										22.50%	22.00%	20.80%
Other Operating Expense Ratio	[1]									10.00%	11.40%	11.60%
Combined Ratio										96.00%	108.30%	87.00%
Corporate
Segment Reporting Information [Line Items]
Net Investment Income	[2]									$ 103,113	$ 89,293	$ 79,451
Total Net Realized and Unrealized Losses	[2]									(30,348)	43,009	9,036
Interest Expense	[2]									(15,485)	(15,447)	(15,435)
Other Income	[2]									4,858	(4,311)	8,565
Merger Transaction Costs	[2]									(3,950)
Income Before Income Taxes	[2]									58,188	112,544	81,617
Income Tax Expense	[2]									(14,336)	(34,312)	(34,783)
Intersegment Elimination
Segment Reporting Information [Line Items]
Commission Expenses	[2]									565	958	1,478
Other Underwriting Income (Expense)	[2]									$ (565)	$ (958)	$ (1,478)

[1] Includes Other Operating Expenses and Other Underwriting Income.

[2] Includes Corporate segment intercompany eliminations.