FHLB Advances, Other Borrowings and Subordinated Notes (Narrative) (Details) $ in Millions	1 Months Ended								3 Months Ended	9 Months Ended	12 Months Ended	24 Months Ended	48 Months Ended	54 Months Ended	62 Months Ended	65 Months Ended	73 Months Ended	77 Months Ended
	May 31, 2020 CAD ($)	Jan. 31, 2020 CAD ($)	May 31, 2019 CAD ($)	Feb. 28, 2019 CAD ($)	Jun. 30, 2018 CAD ($)	Dec. 31, 2017 CAD ($)	Dec. 31, 2015 CAD ($)	Dec. 31, 2014 CAD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2021 USD ($) building Rate	Dec. 31, 2014 USD ($)	Dec. 31, 2015 CAD ($)	Dec. 31, 2017 CAD ($)	Jun. 30, 2018 CAD ($)	Feb. 28, 2019 CAD ($)	May 31, 2019 CAD ($)	Jan. 31, 2020 CAD ($)	May 31, 2020 CAD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jan. 01, 2020	Sep. 18, 2018 USD ($)
Debt Instrument [Line Items]
Notes payable										$ 85,668,000									$ 101,710,000	$ 107,056,000
Short-term borrowings										14,051,000									11,366,000	7,983,000
Other borrowings										63,770,000									65,108,000	65,548,000
Secured borrowings										341,038,000									340,744,000	326,808,000
Subordinated notes										436,811,000									436,506,000	436,385,000
Collateral Pledged
Debt Instrument [Line Items]
Pledged financial instruments										26,100,000
Term Facility
Debt Instrument [Line Items]
Notes payable																							$ 150,000,000
Revolving Credit Facility
Debt Instrument [Line Items]
Notes payable																							50,000,000
Amount outstanding										$ 0
Subordinated debt maturing June 2029
Debt Instrument [Line Items]
Proceeds from the issuance of subordinated notes, net									$ 296,700,000
Base Rate Loan | Base Rate | Term Facility
Debt Instrument [Line Items]
Basis spread on variable rate										0.75%
Base Rate Loan | Base Rate | Revolving Credit Facility
Debt Instrument [Line Items]
Basis spread on variable rate										0.60%
Base Rate Loan | Federal Funds Rate | Revolving Credit Facility
Debt Instrument [Line Items]
Basis spread on variable rate										0.50%
Base Rate Loan | Eurodollar rate | Revolving Credit Facility
Debt Instrument [Line Items]
Basis spread on variable rate										1.00%
Eurodollar Rate Loan | London Interbank Offered Rate (LIBOR) | Term Facility
Debt Instrument [Line Items]
Basis spread on variable rate										1.25%
Eurodollar Rate Loan | London Interbank Offered Rate (LIBOR) | Revolving Credit Facility
Debt Instrument [Line Items]
Basis spread on variable rate										1.35%
Line of Credit
Debt Instrument [Line Items]
Loan agreement with unaffiliated banks																							$ 200,000,000
Revolving Credit Facility
Debt Instrument [Line Items]
Commitment fee | Rate										0.30%
Securities sold under repurchase agreements | Overnight Sweep Collateral
Debt Instrument [Line Items]
Securities sold under agreements to repurchase										$ 14,051,000									11,400,000	8,000,000
Pledged financial instruments										$ 26,058,000
Fixed Rate Promissory Note
Debt Instrument [Line Items]
Number of properties securing debt | building										3
Other borrowings										$ 63,800,000									65,100,000	65,500,000
Fixed interest rate																					3.00%	3.36%
Principal amount of notes																					$ 66,400,000
Secured Debt
Debt Instrument [Line Items]
Secured borrowings										$ 10,000,000
Secured Debt | Receivables Purchase Agreement
Debt Instrument [Line Items]
Fixed interest rate										1.0694%
Proceeds from debt issuance	$ 100	$ 40	$ 70	$ 20	$ 20	$ 10	$ 10	$ 150				$ 160	$ 170	$ 190	$ 210	$ 280	$ 320	$ 420
Secured borrowings										$ 331,000,000									$ 329,900,000	$ 315,100,000
Subordinated Debt | Subordinated debt maturing June 2029
Debt Instrument [Line Items]
Fixed interest rate									4.85%
Principal amount of notes									$ 300,000,000
Subordinated Debt | Subordinated debt maturing June 2024
Debt Instrument [Line Items]
Fixed interest rate											5.00%
Principal amount of notes											$ 140,000,000
Proceeds from debt issuance											$ 139,100,000