Other Investments (Details) - Narrative (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Other Investments [Line Items]
Other investments	$ 1,219,879,000	$ 985,262,000
Aggregate principal amount of payment obligation purchased				150,000,000
Estimated fair value of payment obligation at date of purchase				128,100,000
Fair value of difference between estimated fair value and cost of payment obligation				21,900,000
Coupon rate on outstanding payment obligation				3.50%
Term of payment obligation				11 years 6 months
Payment obligation, fees				2,500,000
Amortization of payment obligation	0	9,400,000	9,500,000
Aggregate face amount of structured transactions purchased				155,000,000
Carrying amount of structured finance transaction entered into by XLIB	147,800,000	147,500,000
Other than temporary declines recorded on other investments during the period	2,700,000	700,000	7,800,000
Minimum
Other Investments [Line Items]
Side pockets - liquidation period of underlying investments	2 years
Maximum
Other Investments [Line Items]
Side pockets - liquidation period of underlying investments	4 years
Alternative funds
Other Investments [Line Items]
Number of strategies employed	4
Number Of Funds Invested In	17
Required notice period for redemption	30 days
Alternative funds | Funds which are subject to lockups and/or gate provisions
Other Investments [Line Items]
Other investments	335,200,000	240,000,000
Alternative funds | Funds held in side pockets
Other Investments [Line Items]
Other investments	$ 25,000,000	$ 28,400,000