Fair Value Disclosures (Analysis of Level 3 Assets and Liabilities Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3	$ 13,600	$ 63,500	$ 49,100	$ 30,900
Transfers of assets from Level 3 to Level 2	13,800	35,700	10,000	44,000
Net gains (losses) on Level 3 assets (realized and unrealized)	11,700	1,500	29,800	17,100
Net gains (losses) on Level 3 liabilities (realized and unrealized)	(4,400)	6,000	26,800	1,500
Excluded assets from unobservable quantitative information	223,700		223,700		$ 228,600
Excluded liabilities from unobservable quantitative information	500		500		$ 4,200
Commercial mortgage-backed securities
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3	2,600		17,000
Transfers of assets from Level 3 to Level 2	4,600
Net gains (losses) on Level 3 assets (realized and unrealized)	(222)	(125)	(1,236)	(1,180)
Net gains (losses) on Level 3 liabilities (realized and unrealized)	70	(35)	(35)	35
Other asset-backed securities
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3	3,900	46,400
Transfers of assets from Level 3 to Level 2		13,500
Net gains (losses) on Level 3 assets (realized and unrealized)	(2,269)	(6,454)	(7,528)	(15,338)
Residential mortgage-backed securities
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3	2,900
Transfers of assets from Level 3 to Level 2		14,600	4,600
Net gains (losses) on Level 3 assets (realized and unrealized)	(66)	(263)	(7,334)	(1,446)
Municipal securities
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Net gains (losses) on Level 3 assets (realized and unrealized)				(1,547)
Loans and other receivables
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3			15,300
Transfers of assets from Level 3 to Level 2				28,300
Net gains (losses) on Level 3 assets (realized and unrealized)	(1,353)	15,261	(2,812)	27,709
Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3				8,100
Net gains (losses) on Level 3 assets (realized and unrealized)	1,057	(2,350)	1,090	(3,280)
Net gains (losses) on Level 3 liabilities (realized and unrealized)	39	0	39	(1)
CDOs and CLOs
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3			8,700	14,000
Transfers of assets from Level 3 to Level 2	3,700
Net gains (losses) on Level 3 assets (realized and unrealized)	(967)	(15,205)	(4,123)	(21,595)
Corporate equity securities
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 3 to Level 2	2,600		2,500
Net gains (losses) on Level 3 assets (realized and unrealized)	11,796	4,344	31,475	3,416
Net gains (losses) on Level 3 liabilities (realized and unrealized)	326	107	365	134
Investments at fair value
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Net gains (losses) on Level 3 assets (realized and unrealized)	2,383	$ 3,964	3,865	$ 12,760
Long-term debt
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of liabilities from Level 2 to Level 3			107,400
Net gains (losses) on Level 3 liabilities (realized and unrealized)	$ 3,004		$ (25,078)