Other Investments - Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 funds strategy	Dec. 31, 2013 strategy funds
Other Investments [Line Items]
Number of strategies employed	4	4
Number of funds invested in	16	16
Required notice period for redemption	30 days
Other investments	$ 1,248,439,000	$ 1,164,630,000
Project Finance Loans:
Aggregate loan value	400,000	15,200,000
Aggregate loan net carrying amount	0	11,100,000
Aggregate principal amount of payment obligation purchased	150,000,000
Estimated fair value of payment obligation at date of purchase	128,100,000
Coupon rate on outstanding payment obligation	3.50%
Term of payment obligation	11 years 6 months
Fair value of difference between estimated fair value and cost of payment obligation	21,900,000
Payment obligation, fees	2,500,000
Other than temporary declines recorded on other investments during the period	12,500,000
Alternative funds | Funds which are subject to lockups and/or gate provisions
Other Investments [Line Items]
Other investments	425,700,000	360,600,000
Alternative funds | Funds held in side pockets
Other Investments [Line Items]
Other investments	24,500,000	24,100,000
Alternative funds | Funds held in side pockets | Minimum
Other Investments [Line Items]
Side pockets - liquidation period of underlying investments	2 years
Alternative funds | Funds held in side pockets | Maximum
Other Investments [Line Items]
Side pockets - liquidation period of underlying investments	4 years
Cypress Point 1 [Member]
Project Finance Loans:
Aggregate face amount of structured transactions purchased	155,000,000
Carrying amount of structured finance transaction entered into by XLIB	136,000,000	148,000,000
Cypress Point 2 [Member]
Project Finance Loans:
Aggregate face amount of structured transactions purchased	81,900,000
Carrying amount of structured finance transaction entered into by XLIB	$ 77,800,000