Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Investment securities
Trading Securities, Realized Gain (Loss), Total	$ 11,700,000	$ 23,700,000	$ 60,200,000	$ 40,500,000
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	16,500,000	49,000,000	5,900,000	82,700,000
Net Realized and Unrealized Gain (Loss) on Trading Securities	28,200,000	72,700,000	66,100,000	123,200,000
Tax on net realized and unrealized gains (loss), trading securities	(1,200,000)	(12,300,000)	(2,500,000)	(28,300,000)
Realized and unrealized gain	27,000,000	60,400,000	63,600,000	94,900,000
Fixed maturity investments.
Investment securities
Trading Securities, Realized Gain (Loss), Total	(9,400,000)	29,000,000	(1,500,000)	69,500,000
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(21,200,000)	(13,400,000)	(88,600,000)	(1,400,000)
Short-term investments
Investment securities
Trading Securities, Realized Gain (Loss), Total	(600,000)	(3,400,000)	200,000	(3,900,000)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	100,000	100,000	0	100,000
Common equity securities
Investment securities
Trading Securities, Realized Gain (Loss), Total	20,500,000	3,900,000	57,700,000	(2,200,000)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	33,600,000	65,000,000	84,200,000	71,400,000
Convertible fixed maturity investments
Investment securities
Trading Securities, Realized Gain (Loss), Total	(200,000)	1,100,000	(700,000)	3,200,000
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	2,300,000	(600,000)	(500,000)	(2,900,000)
Other long-term investments
Investment securities
Trading Securities, Realized Gain (Loss), Total	1,900,000	(7,200,000)	4,800,000	(26,400,000)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	1,700,000	(2,100,000)	10,800,000	15,500,000
Forward Contracts
Investment securities
Trading Securities, Realized Gain (Loss), Total	(500,000)	300,000	(300,000)	300,000
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	$ 0	$ 0	$ 0	$ 0