Investments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) (USD $)	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Mar. 31, 2013 Alternative Investments [Member]	Mar. 31, 2012 Alternative Investments [Member]	Mar. 31, 2013 Alternative Investments [Member]	Mar. 31, 2012 Alternative Investments [Member]	Mar. 31, 2013 Partnerships accounted for under the equity method [Member]	Mar. 31, 2012 Partnerships accounted for under the equity method [Member]	Mar. 31, 2013 Partnerships accounted for under the equity method [Member]	Mar. 31, 2012 Partnerships accounted for under the equity method [Member]
Schedule of Equity Method Investments [Line Items]
Net investment income									$ 46,800,000	$ 54,000,000	$ 255,000,000	$ 90,100,000
Realized (losses) gains									(22,100,000)	234,600,000	599,700,000	985,300,000
Net change in unrealized depreciation									378,800,000	53,400,000	(18,900,000)	(434,000,000)
Net income									403,500,000	342,000,000	835,800,000	641,400,000
Net investment income earned	$ 34,003,000	$ 34,006,000	$ 66,873,000	$ 66,634,000	$ 3,900,000	$ 3,000,000	$ 7,500,000	$ 5,000,000