Segment Reporting Disclosures (Company's Segment Disclosures) (Details) (USD $)	3 Months Ended				12 Months Ended	3 Months Ended
	Mar. 31, 2012 segment		Mar. 31, 2011		Dec. 31, 2012 Accounting Standards Update 2010-26 [Member]	Mar. 31, 2012 Excess And Surplus Lines [Member]		Mar. 31, 2011 Excess And Surplus Lines [Member]		Mar. 31, 2012 Specialty Admitted [Member]		Mar. 31, 2011 Specialty Admitted [Member]		Mar. 31, 2012 London Insurance Market [Member]		Mar. 31, 2011 London Insurance Market [Member]		Mar. 31, 2012 Other Insurance (Discontinued Lines) [Member]		Mar. 31, 2011 Other Insurance (Discontinued Lines) [Member]		Mar. 31, 2012 Investing [Member]		Mar. 31, 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	$ 648,618,000		$ 590,783,000			$ 222,929,000		$ 201,371,000		$ 148,122,000		$ 134,321,000		$ 277,566,000		$ 255,001,000		$ 1,000		$ 90,000		$ 0		$ 0
Net written premiums	581,166,000		519,012,000			192,913,000		175,537,000		140,552,000		127,239,000		247,700,000		216,139,000		1,000		97,000		0		0
Earned premiums	529,596,000		463,111,000			199,378,000		181,057,000		133,475,000		122,476,000		196,742,000		159,483,000		1,000		95,000		0		0
Losses and loss adjustment expenses, Current accident year	(352,506,000)		(388,853,000)			(128,067,000)		(122,507,000)		(92,693,000)		(78,470,000)		(131,746,000)		(187,876,000)		0		0		0		0
Losses and loss adjustment expenses, Prior accident years	63,985,000		74,525,000			30,587,000		56,792,000		4,326,000		198,000		21,465,000		12,634,000		7,607,000		4,901,000		0		0
Prospective adoption of ASU 2010-26	(20,288,000)	[1]				(8,487,000)	[1]			(5,764,000)	[1]			(6,037,000)	[1]			0	[1]			0	[1]
All other expenses	(218,409,000)					(85,857,000)				(57,266,000)				(75,003,000)				(283,000)				0
Underwriting, acquisition and insurance expenses	(238,697,000)		(202,350,000)					(85,920,000)				(49,473,000)				(66,725,000)				(232,000)				0
Underwriting profit (loss)	2,378,000		(53,567,000)			7,554,000		29,422,000		(17,922,000)		(5,269,000)		5,421,000		(82,484,000)		7,325,000		4,764,000		0		0
Net investment income	79,794,000		70,099,000			0		0		0		0		0		0		0		0		79,794,000		70,099,000
Net realized investment gains	11,909,000		11,240,000			0		0		0		0		0		0		0		0		11,909,000		11,240,000
Other revenues (insurance)	14,831,000		9,186,000			0		0		10,448,000		9,186,000		4,383,000		0		0		0		0		0
Other expenses (insurance)	(12,175,000)		(11,748,000)			0		0		(11,201,000)		(11,740,000)		(974,000)		(8,000)		0		0		0		0
Segment profit (loss)	96,737,000		25,210,000			7,554,000		29,422,000		(18,675,000)		(7,823,000)		8,830,000		(82,492,000)		7,325,000		4,764,000		91,703,000		81,339,000
Other revenues (non-insurance)	97,005,000		67,958,000
Other expenses (non-insurance)	(88,229,000)		(56,747,000)
Amortization of intangible assets	(8,804,000)		(6,008,000)
Interest expense	(22,167,000)		(18,962,000)
Income before income taxes	74,542,000		11,451,000
U.S. GAAP combined ratio	100.00%	[2]	112.00%	[2]		96.00%	[2]	84.00%	[2]	113.00%	[2]	104.00%	[2]	97.00%	[2]	152.00%	[2]
U.S. GAAP combined ratio - not meaniningful																		NM	[2],[3]	NM	[2],[3]
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 three months ended March 31, 2012 included $20.3 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] 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.

[3] NM – Ratio is not meaningful.