Loans (Narrative) (Details)		3 Months Ended			9 Months Ended		12 Months Ended
		Mar. 31, 2020 USD ($) security	Sep. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Oct. 29, 2020 USD ($)	Jan. 01, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale					$ 1,850,000
Loans available for sale, unpaid principal amount					1,850,000,000
Gains (losses) recognized from changes in fair value					(3,100,000)	$ 1,600,000
Changes in fair value gain (loss) of loans, credit weaknesses amount					$ 490,000
Prepayments percentage, loans					15.00%
Loans Receivable, Gross, Commercial, Mortgage			$ 778,200,000	$ 518,300,000	$ 2,488,760,000	778,200,000
Gain (Loss) on Securitization of Financial Assets			14,200,000	11,200,000
Transfers of Financial Assets Accounted for as Sale, Initial Fair Value of Assets Obtained as Proceeds			$ 51,600,000	$ 41,600,000		51,600,000
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Discount Rate			4.12%	4.75%
Interest which would have been earned on loans classified as non-accrual					459,000	$ 356,000
Commitments to lend additional funds to loan customers whose terms have been modified in troubled debt restructurings					0		$ 0
Non-accrual loans, income					0
Nonaccrual loans, Income Reversed					361,000
Loans acquired with deteriorated credit quality					0		0
Financing Receivable, Troubled Debt Restructuring, Subsequent Default, Number of Contracts | security		1
Financing Receivable, Modifications, Post-Modification Recorded Investment					1,661,000		2,084,000
Financing Receivable, Troubled Debt Restructured Loans, Reserves					479,000
Total loans, gross					2,482,452,000		$ 1,814,488,000
Financing Receivable Allowance For Credit Losses On Off-Balance Sheet Credit					$ 570,000,000
Equity Securities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, advanced rate calculation, percentage					50
Mutual Fund [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, advanced rate calculation, percentage					50
Debt Securities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, advanced rate calculation, percentage					80
ASU 2016-13 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease)		$ 2,600,000
Financing Receivable Allowance For Off-Balance Credit Losses Period Increase		569,000
Financing Receivable Allowance For Credit Losses On Off-Balance Sheet Credit					$ 570,000				$ 569,000
Commercial Mortgage - Securitization [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage					0
Total loans, gross					1,600,000,000
SBLOC [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gains (losses) recognized from changes in fair value					0
Loans Receivable, Gross, Commercial, Mortgage					1,072,887,000
Vehicles Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans, gross		$ 15,300,000			1,700,000
Vehicles Loan [Member] | Subsequent Event [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans, gross								$ 690,000
Advisor Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage					26,986,000
Total loans, gross	[1]				$ 26,600,000
Loan Amount, Loan-To-Value Ratio					70
Paycheck Protection Program Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans, gross					$ 207,900,000
Government Guaranteed Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans, gross					$ 334,700,000

[1] In 2020, the Company began originating loans to investment advisors for purposes of debt refinance, acquisition of another firm or internal succession. Maximum loan amounts are subject to loan-to-value ratios of 70%, based on third party business appraisals, but may be increased depending upon the debt service coverage ratio. Personal guarantees and blanket business liens are obtained as appropriate.