Investments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2014	Jun. 30, 2014	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2014	Jun. 30, 2014
Schedule of Equity Method Investments [Line Items]
Net investment income earned	$ 32,061		$ 34,292		$ 91,208		$ 106,600
Alternative Investments [Member]
Schedule of Equity Method Investments [Line Items]
Net investment income earned	$ 1,300		$ 3,900		$ (800)		$ 12,700
Investments accounted for under the equity method [Member]
Schedule of Equity Method Investments [Line Items]
Net Investment Income		$ 44,100		$ 81,300		$ 139,600		$ 167,000
Realized gains (losses)		385,200		(26,100)		977,700		171,500
Net change in unrealized (depreciation) appreciation		(222,200)		628,600		(1,089,000)		1,471,000
Net income		$ 207,100		$ 683,800		$ 28,300		$ 1,809,500