Investments in Affiliates (Details) - Narrative (USD $)			12 Months Ended						12 Months Ended				3 Months Ended	12 Months Ended				3 Months Ended	12 Months Ended				3 Months Ended							12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Maximum	Dec. 31, 2012 Minimum	Dec. 31, 2012 Investment Fund Affiliates [Member]		Dec. 31, 2011 Investment Fund Affiliates [Member]		Dec. 31, 2012 Investment Fund Affiliates [Member] Maximum	Dec. 31, 2012 Investment Fund Affiliates [Member] Minimum	Dec. 31, 2012 Operating affiliates	Dec. 31, 2011 Operating affiliates	Dec. 31, 2010 Primus Guaranty Ltd [Member]	Dec. 31, 2012 Other Strategic Operating Affiliates [Member] investment_companies	Dec. 31, 2011 Other Strategic Operating Affiliates [Member]	Dec. 31, 2012 ARX Holding Corporation [Member]	Dec. 31, 2011 ARX Holding Corporation [Member]	Jun. 30, 2010 Itau Xl Seguros Corporativos Sa [Member]	Dec. 31, 2012 Investment Manager Affiliates [Member]	Dec. 31, 2011 Investment Manager Affiliates [Member]		Dec. 31, 2010 Investment Manager Affiliates [Member]	Sep. 30, 2011 Finisterre [Member]	Dec. 31, 2012 Subject To Redemption Restrictions [Member]	Dec. 31, 2011 Subject To Redemption Restrictions [Member]	Dec. 31, 2012 Redemption Restriction Gate [Member]	Dec. 31, 2011 Redemption Restriction Gate [Member]	Dec. 31, 2012 Redemption Restriction Side Pocket [Member]	Dec. 31, 2011 Redemption Restriction Side Pocket [Member]	Dec. 31, 2012 Investments In Common Stock Of Corporations [Member]	Dec. 31, 2012 Investments In Other Types Of Legal Structures [Member]
Schedule of Equity Method Investments [Line Items]
Threshold for signicant influence, percentage																														20.00%	3.00%
Alternative investments, timelag for reporting									3 months	1 month	3 months
Carrying value of equity method investment, operating affiliates	$ 1,126,875,000	$ 1,052,729,000			$ 845,197,000	[1]	$ 768,851,000	[1]			$ 281,678,000	$ 283,878,000		$ 192,792,000	$ 153,955,000				$ 88,154,000	$ 129,191,000	[2]			$ 302,400,000	$ 419,400,000	$ 1,400,000	$ 0	$ 39,700,000	$ 30,700,000
Ownership percentage remaining					4.70%		4.10%						9.80%			39.70%	45.70%
Ownership percentage before sale													39.70%
Sale Of Stock Percentage Sold													76.00%					49.90%
Sales proceeds on sale of affiliate													51,600,000										35,000,000
Distributions received from investment manager affiliates																			58,300,000	23,200,000
Gain on disposal of affiliate													51,600,000										25,300,000
Other than temporary impairments in investment management affiliates																			0	600,000		4,400,000
Side pockets - liquidation period of underlying investments			4 years	2 years
Number of interests in operating affiliates purchased														2
Cost of operating affiliate interests acquired														$ 20,900,000

[1] The Company generally records its alternative fund affiliates on a one month lag and its private investments fund affiliates on a three month lag.

[2] During the year ended December 31, 2012 and 2011 the Company received distributions from its Investments Manager Affiliates of approximately $58.3 million and $23.2 million respectively. During 2011 the Company sold its interests in Finisterre for total proceeds of $35 million as explained below at "Investment Manager Affiliate".