Segment Reporting Disclosures (Company's Segment Disclosures) (Details) (USD $)	3 Months Ended				9 Months Ended				12 Months Ended	3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended				3 Months Ended			9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012 segment		Sep. 30, 2011		Dec. 31, 2012 Accounting Standards Update 2010-26 [Member]	Sep. 30, 2012 Excess And Surplus Lines [Member]		Sep. 30, 2011 Excess And Surplus Lines [Member]		Sep. 30, 2012 Excess And Surplus Lines [Member]		Sep. 30, 2011 Excess And Surplus Lines [Member]		Sep. 30, 2012 Specialty Admitted [Member]		Sep. 30, 2011 Specialty Admitted [Member]		Sep. 30, 2012 Specialty Admitted [Member]		Sep. 30, 2011 Specialty Admitted [Member]		Sep. 30, 2012 London Insurance Market [Member]		Sep. 30, 2011 London Insurance Market [Member]		Sep. 30, 2012 London Insurance Market [Member]		Sep. 30, 2011 London Insurance Market [Member]		Sep. 30, 2012 Other Insurance (Discontinued Lines) [Member]		Sep. 30, 2011 Other Insurance (Discontinued Lines) [Member]		Sep. 30, 2012 Other Insurance (Discontinued Lines) [Member]		Sep. 30, 2011 Other Insurance (Discontinued Lines) [Member]		Sep. 30, 2012 Investing [Member]		Sep. 30, 2011 Investing [Member]	Sep. 30, 2012 Investing [Member]		Sep. 30, 2011 Investing [Member]
Segment Reporting Information [Line Items]
Number of segments of the specialty insurance marketplace in which the Company operates					3
Gross premium volume	$ 610,833,000		$ 584,635,000		$ 1,906,373,000		$ 1,772,611,000			$ 253,014,000		$ 236,639,000		$ 705,849,000		$ 663,989,000		$ 167,747,000		$ 153,753,000		$ 496,019,000		$ 431,604,000		$ 190,071,000		$ 194,210,000		$ 704,511,000		$ 676,893,000		$ 1,000		$ 33,000		$ (6,000)		$ 125,000		$ 0		$ 0	$ 0		$ 0
Net written premiums	539,624,000		524,678,000		1,687,404,000		1,574,378,000			211,538,000		200,658,000		597,742,000		570,243,000		157,894,000		147,169,000		467,722,000		410,700,000		170,193,000		176,829,000		621,947,000		593,440,000		(1,000)		22,000		(7,000)		(5,000)		0		0	0		0
Earned premiums	530,537,000		509,203,000		1,573,189,000		1,462,515,000			195,478,000		189,695,000		584,524,000		557,958,000		153,009,000		136,783,000		431,179,000		390,623,000		182,052,000		182,710,000		557,493,000		513,947,000		(2,000)		15,000		(7,000)		(13,000)		0		0	0		0
Losses and loss adjustment expenses, current accident year	(372,740,000)		(405,575,000)		(1,073,238,000)		(1,177,848,000)			(134,504,000)		(147,992,000)		(390,254,000)		(413,049,000)		(116,044,000)		(112,607,000)		(310,115,000)		(285,401,000)		(122,192,000)		(144,976,000)		(372,869,000)		(479,398,000)		0		0		0		0		0		0	0		0
Losses and loss adjustment expenses, prior accident years	69,281,000		98,943,000		260,164,000		250,205,000			51,310,000		60,475,000		132,583,000		169,916,000		11,504,000		5,973,000		27,747,000		7,942,000		32,744,000		34,721,000		118,994,000		69,960,000		(26,277,000)		(2,226,000)		(19,160,000)		2,387,000		0		0	0		0
Prospective adoption of ASU 2010-26	(6,534,000)	[1]			(41,093,000)	[2]				(2,125,000)	[1]			(16,652,000)	[2]			(2,600,000)	[1]			(12,863,000)	[2]			(1,809,000)	[1]			(11,578,000)	[2]			0	[1]			0	[2]			0	[1]		0	[2]
All other expenses	(225,307,000)				(654,229,000)					(88,093,000)				(255,295,000)				(60,008,000)				(171,111,000)				(76,820,000)				(226,770,000)				(386,000)				(1,053,000)				0			0
Underwriting, acquisition and insurance expenses	(231,841,000)		(202,316,000)		(695,322,000)		(601,511,000)					(81,563,000)				(250,415,000)				(51,695,000)				(148,957,000)				(69,731,000)				(202,291,000)				673,000				152,000				0			0
Underwriting profit (loss)	(4,763,000)		255,000		64,793,000		(66,639,000)			22,066,000		20,615,000		54,906,000		64,410,000		(14,139,000)		(21,546,000)		(35,163,000)		(35,793,000)		13,975,000		2,724,000		65,270,000		(97,782,000)		(26,665,000)		(1,538,000)		(20,220,000)		2,526,000		0		0	0		0
Net investment income	64,438,000		62,199,000		207,834,000		196,551,000			0		0		0		0		0		0		0		0		0		0		0		0		0		0		0		0		64,438,000		62,199,000	207,834,000		196,551,000
Net realized investment gains	5,231,000		12,839,000		25,356,000		25,423,000			0		0		0		0		0		0		0		0		0		0		0		0		0		0		0		0		5,231,000		12,839,000	25,356,000		25,423,000
Other revenues (insurance)	11,759,000		10,061,000		40,818,000		31,622,000			0		0		0		0		11,536,000		10,061,000		36,065,000		31,622,000		223,000		0		4,753,000		0		0		0		0		0		0		0	0		0
Other expenses (insurance)	(13,151,000)		(2,999,000)		(37,906,000)		(27,374,000)			0		0		0		0		(12,181,000)		(3,046,000)		(35,184,000)		(27,374,000)		(970,000)		47,000		(2,722,000)		0		0		0		0		0		0		0	0		0
Segment profit (loss)	63,514,000		82,355,000		300,895,000		159,583,000			22,066,000		20,615,000		54,906,000		64,410,000		(14,784,000)		(14,531,000)		(34,282,000)		(31,545,000)		13,228,000		2,771,000		67,301,000		(97,782,000)		(26,665,000)		(1,538,000)		(20,220,000)		2,526,000		69,669,000		75,038,000	233,190,000		221,974,000
Other revenues (non-insurance)	153,810,000		81,786,000		344,960,000		228,739,000
Other expenses (non-insurance)	(132,188,000)		(67,303,000)		(305,556,000)		(190,896,000)
Amortization of intangible assets	(7,959,000)		(6,023,000)		(25,078,000)		(17,586,000)
Interest expense	(24,692,000)		(23,656,000)		(69,068,000)		(64,516,000)
Income before income taxes	52,485,000		67,159,000		246,153,000		115,324,000
U.S. GAAP combined ratio	101.00%	[3]	100.00%	[3]	96.00%	[3]	105.00%	[3]		89.00%	[3]	89.00%	[3]	91.00%	[3]	88.00%	[3]	109.00%	[3]	116.00%	[3]	108.00%	[3]	109.00%	[3]	92.00%	[3]	99.00%	[3]	88.00%	[3]	119.00%	[3]
U.S. GAAP combined ratio - not meaniningful																																		NM	[3],[4]	NM	[3],[4]	NM	[3],[4]	NM	[3],[4]
Costs expected to be recognized in income, effect of adoption									$ 43,000,000

[1] Effective January 1, 2012, the Company prospectively adopted Financial Accounting Standards Board (FASB) Accounting Standards Update (ASU) No. 2010-26, Accounting for Costs Associated with Acquiring or Renewing Insurance Contracts. At December 31, 2011, deferred policy acquisition costs included approximately $43 million of costs that no longer met the criteria for deferral as of January 1, 2012 and will be recognized into income primarily over the first nine months of 2012, consistent with policy terms. The quarter ended September 30, 2012 included $6.5 million of underwriting, acquisition and insurance expenses that were deferred as of December 31, 2011 and no longer met the criteria for deferral as of January 1, 2012.

[2] Effective January 1, 2012, the Company prospectively adopted FASB ASU No. 2010-26, Accounting for Costs Associated with Acquiring or Renewing Insurance Contracts. At December 31, 2011, deferred policy acquisition costs included approximately $43 million of costs that no longer met the criteria for deferral as of January 1, 2012 and will be recognized into income primarily over the first nine months of 2012, consistent with policy terms. The nine months ended September 30, 2012 included $41.1 million of underwriting, acquisition and insurance expenses that were deferred as of December 31, 2011 and no longer met the criteria for deferral as of January 1, 2012.

[3] The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.

[4] NM – Ratio is not meaningful.