Bank Loans (Aging Of The Recorded Investment In Past Due Loans) (Details) - USD ($)	Mar. 31, 2018		Dec. 31, 2017
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	$ 14,372,000		$ 19,961,000
Current Balance	7,140,289,000		6,994,460,000
Recorded Investment in Loans, Total	7,154,661,000		7,014,421,000
Non-Accrual	14,888,000	[1]	20,090,000	[2]
Restructured	9,106,000	[1]	9,123,000	[2]
Total	23,994,000	[1]	29,213,000	[2]
Loans past due 90 days and still accruing interest	0		0
30 - 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	14,372,000		19,961,000
Residential Real Estate [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	2,330,000		7,892,000
Current Balance	2,631,739,000		2,585,684,000
Recorded Investment in Loans, Total	2,634,069,000		2,593,576,000
Restructured	170,000	[1]	171,000	[2]
Total	170,000	[1]	171,000	[2]
Residential Real Estate [Member] | 30 - 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	2,330,000		7,892,000
Commercial And Industrial [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	11,883,000		11,883,000
Current Balance	2,541,788,000		2,426,055,000
Recorded Investment in Loans, Total	2,553,671,000		2,437,938,000
Non-Accrual	14,702,000	[1]	19,904,000	[2]
Restructured	8,936,000	[1]	8,952,000	[2]
Total	23,638,000	[1]	28,856,000	[2]
Commercial And Industrial [Member] | 30 - 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	11,883,000		11,883,000
Securities-Based Loans [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Current Balance	1,809,281,000		1,819,206,000
Recorded Investment in Loans, Total	1,809,281,000		1,819,206,000
Commercial Real Estate [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Current Balance	101,591,000		116,258,000
Recorded Investment in Loans, Total	101,591,000		116,258,000
Consumer [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due			2,000
Current Balance	24,699,000		24,506,000
Recorded Investment in Loans, Total	24,699,000		24,508,000
Non-Accrual	2,000	[1]	2,000	[2]
Total	2,000	[1]	2,000	[2]
Consumer [Member] | 30 - 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due			2,000
Home Equity Lines Of Credit [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	159,000		184,000
Current Balance	14,854,000		14,855,000
Recorded Investment in Loans, Total	15,013,000		15,039,000
Non-Accrual	184,000	[1]	184,000	[2]
Total	184,000	[1]	184,000	[2]
Home Equity Lines Of Credit [Member] | 30 - 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	159,000		184,000
Construction And Land [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Current Balance	16,337,000		7,896,000
Recorded Investment in Loans, Total	$ 16,337,000		$ 7,896,000

[1] There were no loans past due 90 days and still accruing interest at March 31, 2018.

[2] There were no loans past due 90 days and still accruing interest at December 31, 2017.