Investments in Affiliates - Investment Fund Affiliates (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Schedule of Equity Method Investments [Line Items]
Carrying value of equity method investments, investment fund affiliates	$ 1,637,620		$ 1,370,943
Equity in net income (loss) in operating affiliates	203,034		258,195		112,391
Investment fund affiliates
Schedule of Equity Method Investments [Line Items]
Carrying value of equity method investments, investment fund affiliates	1,219,138	[1]	1,042,072	[1]
Equity in net income (loss) in operating affiliates	95,816	[1]	138,391	[1]
Weighted average XL percentage ownership	6.40%	[1]	5.30%	[1]
Combined funds/ estimated total net assets	19,177,932	[1],[2]	19,766,826	[1],[2]
Investment fund affiliates | Alternative funds
Schedule of Equity Method Investments [Line Items]
Carrying value of equity method investments, investment fund affiliates	1,023,160	[1]	867,502	[1]
Equity in net income (loss) in operating affiliates	67,037	[1]	108,449	[1]
Weighted average XL percentage ownership	5.70%	[1]	4.60%	[1]
Combined funds/ estimated total net assets	18,072,156	[1],[2]	18,876,011	[1],[2]
Investment fund affiliates | Arbitrage
Schedule of Equity Method Investments [Line Items]
Carrying value of equity method investments, investment fund affiliates	134,400	[1]	158,096	[1]
Equity in net income (loss) in operating affiliates	12,060	[1]	4,358	[1]
Weighted average XL percentage ownership	11.00%	[1]	3.70%	[1]
Combined funds/ estimated total net assets	1,224,526	[1],[2]	4,328,748	[1],[2]
Investment fund affiliates | Directional
Schedule of Equity Method Investments [Line Items]
Carrying value of equity method investments, investment fund affiliates	536,470	[1]	434,652	[1]
Equity in net income (loss) in operating affiliates	32,196	[1]	42,492	[1]
Weighted average XL percentage ownership	9.20%	[1]	8.30%	[1]
Combined funds/ estimated total net assets	5,865,247	[1],[2]	5,235,690	[1],[2]
Investment fund affiliates | Event driven
Schedule of Equity Method Investments [Line Items]
Carrying value of equity method investments, investment fund affiliates	300,168	[1],[3]	274,380	[1],[3]
Equity in net income (loss) in operating affiliates	21,030	[1],[3]	61,551	[1],[3]
Weighted average XL percentage ownership	3.10%	[1],[3]	2.90%	[1],[3]
Combined funds/ estimated total net assets	9,703,973	[1],[2],[3]	9,309,163	[1],[2],[3]
Investment fund affiliates | Multi-style
Schedule of Equity Method Investments [Line Items]
Carrying value of equity method investments, investment fund affiliates	52,122	[1]	374	[1]
Equity in net income (loss) in operating affiliates	1,751	[1]	48	[1]
Weighted average XL percentage ownership	4.10%	[1]	15.50%	[1]
Combined funds/ estimated total net assets	1,278,410	[1],[2]	2,410	[1],[2]
Investment fund affiliates | Private investment funds
Schedule of Equity Method Investments [Line Items]
Carrying value of equity method investments, investment fund affiliates	195,978	[1]	174,570	[1]
Equity in net income (loss) in operating affiliates	28,779	[1]	29,942	[1]
Weighted average XL percentage ownership	17.70%	[1]	19.60%	[1]
Combined funds/ estimated total net assets	$ 1,105,776	[1],[2]	$ 890,815	[1],[2]

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

[2] Total estimated net assets are generally as at November 30 for alternative fund affiliates and September 30 for private investment fund affiliates.

[3] The Company accounts for its investment in certain funds where the ownership percentage is less than three percent using the equity method, where the Company has significant influence over the related investment management company.