Fair Value Disclosures (Analysis of Level 3 Assets and Liabilities Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3	$ 107,100	$ 98,400	$ 155,900	$ 155,000
Transfers of assets from Level 3 to Level 2	62,700	96,400	92,900	87,000
Transfers of liabilities from Level 3 to Level 2				3,000
Net gains (losses) on Level 3 assets (realized and unrealized)	(69,000)	(109,300)	(87,600)	557,000
Net gains (losses) on Level 3 liabilities (realized and unrealized)	300	4,900	(9,500)	(1,900)
Excluded assets from unobservable quantitative information	290,500		290,500		$ 280,600
Excluded liabilities from unobservable quantitative information	500		500		$ 600
Non-Agency Residential Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3		30,000	21,700	20,300
Transfers of assets from Level 3 to Level 2	19,500	23,000	16,700	15,100
Commercial Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3		7,700		10,200
Transfers of assets from Level 3 to Level 2		5,500		3,500
Net gains (losses) on Level 3 assets (realized and unrealized)	(1,140)	(789)	(2,589)	(1,124)
Other Asset-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3	30,700	2,100	28,000	2,100
Transfers of assets from Level 3 to Level 2	16,000	5,400
Net gains (losses) on Level 3 assets (realized and unrealized)	(7,284)	(19)	(202)	(258)
Residential Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3	19,300
Net gains (losses) on Level 3 assets (realized and unrealized)	(4,915)	(1,820)	(8,337)	(3,735)
Loans and other receivables [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3	15,900	1,000	20,200	13,900
Transfers of assets from Level 3 to Level 2			16,800	5,300
Net gains (losses) on Level 3 assets (realized and unrealized)	(7,792)	(748)	(13,376)	(5,795)
Corporate Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3		8,200		10,400
Net gains (losses) on Level 3 assets (realized and unrealized)	474	(532)	(2,378)	(796)
Collateralized Debt Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3		48,000	30,600	27,300
Transfers of assets from Level 3 to Level 2		54,400	22,300	52,300
Net gains (losses) on Level 3 assets (realized and unrealized)	1,797	(5,120)	(20,455)	(17,229)
Corporate Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3	22,000	1,400		1,600
Transfers of assets from Level 3 to Level 2		7,000		10,300
Net gains (losses) on Level 3 assets (realized and unrealized)	(927)	8,030	1,571	7,066
Investments at Fair Value [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3			26,100	69,200
Transfers of assets from Level 3 to Level 2			16,100
Net gains (losses) on Level 3 assets (realized and unrealized)	(1,375)	3,766	59,242	4,311
Other Secured Financings [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Net gains (losses) on Level 3 liabilities (realized and unrealized)	$ (70)	$ 0	$ (76)	$ 0