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 ($)	Sep. 30, 2020 USD ($) building Rate	Sep. 30, 2019 USD ($)	Dec. 31, 2014 USD ($)	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 ($)	Apr. 01, 2020 USD ($)	Mar. 31, 2020	Dec. 31, 2019 USD ($)	Sep. 18, 2018 USD ($)
Debt Instrument [Line Items]
Notes payable										$ 107,056,000	$ 128,433,000											$ 123,090,000	$ 150,000,000.0
Short-term borrowings										7,983,000	9,677,000											20,520,000
Other borrowings										65,548,000	46,774,000											46,447,000
Secured borrowings										326,808,000	225,604,000											228,117,000
Subordinated notes										436,385,000	435,979,000											436,095,000
Proceeds from the issuance of subordinated notes, net										0	296,617,000
Term Facility
Debt Instrument [Line Items]
Notes payable										107,100,000													150,000,000.0
Revolving Credit Facility
Debt Instrument [Line Items]
Notes payable																							50,000,000.0
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																							$ 200,000,000.0
Revolving Credit Facility
Debt Instrument [Line Items]
Commitment fee | Rate										0.30%
Securities sold under repurchase agreements
Debt Instrument [Line Items]
Securities sold under agreements to repurchase											9,700,000											20,500,000
Securities sold under repurchase agreements | Overnight Sweep Collateral
Debt Instrument [Line Items]
Securities sold under agreements to repurchase										$ 7,983,000
Pledged financial instruments										$ 18,595,000
Fixed Rate Promissory Note
Debt Instrument [Line Items]
Number of properties securing debt | building										3
Other borrowings										$ 65,500,000	46,800,000											46,400,000
Fixed interest rate																				3.00%	3.36%
Principal amount of notes																				$ 66,400,000
Secured Debt
Debt Instrument [Line Items]
Secured borrowings										$ 11,700,000
Secured Debt | Receivables Purchase Agreement
Debt Instrument [Line Items]
Fixed interest rate										1.2647%
Proceeds from debt issuance	$ 100	$ 40	$ 70	$ 20	$ 20	$ 10	$ 10	$ 150					$ 160	$ 170	$ 190	$ 210	$ 280	$ 320	$ 420
Secured borrowings										$ 315,100,000	$ 211,400,000											$ 215,500,000
Subordinated Debt | Subordinated debt maturing June 2029
Debt Instrument [Line Items]
Fixed interest rate									4.85%
Principal amount of notes									$ 300,000,000.0
Subordinated Debt | Subordinated debt maturing June 2024
Debt Instrument [Line Items]
Fixed interest rate												5.00%
Principal amount of notes												$ 140,000,000.0
Proceeds from debt issuance												$ 139,100,000