Investments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Mar. 31, 2014
Schedule of Equity Method Investments [Line Items]
Net investment income earned	$ 32,230		$ 36,774		$ 59,147		$ 72,308
Alternative Investments [Member]
Schedule of Equity Method Investments [Line Items]
Net investment income earned	$ 1,400		$ 3,600		$ (2,100)		$ 8,800
Investments accounted for under the equity method [Member]
Schedule of Equity Method Investments [Line Items]
Net Investment Income		$ 8,500		$ 22,800		$ 95,600		$ 85,700
Realized gains		279,400		74,200		592,500		197,600
Net change in unrealized (depreciation) appreciation		(223,400)		207,600		(866,900)		842,400
Net income (loss)		$ 64,500		$ 304,600		$ (178,800)		$ 1,125,700