Investments in Affiliates (Details) - Investment Fund Affiliates (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Schedule of Equity Method Investments [Line Items]
Carrying value of equity method investments, investment fund affiliates	$ 1,126,875		$ 1,052,729
Equity in net income (loss) in investment fund affiliates	114,314		103,039		172,474
Investment Fund Affiliates [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value of equity method investments, investment fund affiliates	845,197	[1]	768,851	[1]
Equity in net income (loss) in investment fund affiliates	58,504	[1]	26,253	[1]
Weighted average XL percentage ownership	4.70%		4.10%
Combined funds/ estimated total net assets	18,124,965	[2]	18,593,763	[2]
Investment Fund Affiliates [Member] | Alternative funds
Schedule of Equity Method Investments [Line Items]
Carrying value of equity method investments, investment fund affiliates	627,180		572,129
Equity in net income (loss) in investment fund affiliates	34,966		10,376
Weighted average XL percentage ownership	3.70%		3.90%
Combined funds/ estimated total net assets	16,849,422	[2]	14,773,266	[2]
Investment Fund Affiliates [Member] | Arbitrage
Schedule of Equity Method Investments [Line Items]
Carrying value of equity method investments, investment fund affiliates	131,037		123,969
Equity in net income (loss) in investment fund affiliates	7,149		1,291
Weighted average XL percentage ownership	3.70%		3.40%
Combined funds/ estimated total net assets	3,517,404	[2]	3,678,904	[2]
Investment Fund Affiliates [Member] | Directional
Schedule of Equity Method Investments [Line Items]
Carrying value of equity method investments, investment fund affiliates	297,535		217,779
Equity in net income (loss) in investment fund affiliates	14,791		9,154
Weighted average XL percentage ownership	5.90%		6.00%
Combined funds/ estimated total net assets	5,057,381	[2]	3,619,272	[2]
Investment Fund Affiliates [Member] | Event driven
Schedule of Equity Method Investments [Line Items]
Carrying value of equity method investments, investment fund affiliates	198,282	[3]	229,956
Equity in net income (loss) in investment fund affiliates	13,125	[3]	(60)
Weighted average XL percentage ownership	2.40%		3.10%
Combined funds/ estimated total net assets	8,272,227	[2]	7,472,680	[2]
Investment Fund Affiliates [Member] | Multi-style
Schedule of Equity Method Investments [Line Items]
Carrying value of equity method investments, investment fund affiliates	326		425
Equity in net income (loss) in investment fund affiliates	(99)		(9)
Weighted average XL percentage ownership	13.50%		17.60%
Combined funds/ estimated total net assets	2,410	[2]	2,410	[2]
Investment Fund Affiliates [Member] | Private investment funds
Schedule of Equity Method Investments [Line Items]
Carrying value of equity method investments, investment fund affiliates	218,017		196,722
Equity in net income (loss) in investment fund affiliates	23,538		15,877
Weighted average XL percentage ownership	17.10%		5.10%
Combined funds/ estimated total net assets	$ 1,275,543	[2]	$ 3,820,497	[2]

[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] Total estimated net assets are generally at November 30 and September 30, respectively.

[3] The Company accounts for its investment in certain funds with an ownership percentage of less than three percent where the Company has significant influence over the related investments management company.