Mortgage Banking Activities, Mortgage Servicing Rights (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Summary of Mortgage Servicing Rights [Abstract]
Number of residential mortgage loans serviced for others	111,008		111,008		126,828
Outstanding principal balance of residential mortgage loans serviced for others	$ 17,116,204,000		$ 17,116,204,000		$ 20,727,106,000
Weighted average interest rate	3.91%		3.91%		3.98%
Remaining term (in months)			281		289
Servicing Asset at Fair Value, Amount [Roll Forward]
Beginning balance	$ 97,971,000	$ 208,308,000	$ 201,886,000	$ 259,254,000	$ 259,254,000
Additions	7,424,000	9,882,000	21,330,000	24,821,000
Disposals	0	0	(10,801,000)	0
Change in fair value due to principal payments	(11,192,000)	(11,936,000)	(28,971,000)	(27,600,000)
Change in fair value due to market assumption changes	3,441,000	(12,593,000)	(85,800,000)	(62,814,000)
Ending balance	$ 97,644,000	$ 193,661,000	$ 97,644,000	$ 193,661,000	$ 201,886,000
Servicing Assets at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Discount rate - risk-free rate plus a market premium (in hundredths)			9.15%		9.81%
Prepayment rate - based upon loan interest rate, original term and loan type, minimum			9.90%		8.28%
Prepayment rate - based upon loan interest rate, original term and loan type, maximum			24.02%		16.05%
Loan servicing costs - annually per loan based upon loan type, performing, minimum (in dollars per loan)			$ 69		$ 68
Loan servicing costs - annually per loan based upon loan type, performing, maximum (in dollars per loan)			94		94
Loan servicing costs - annually per loan based upon loan type, delinquent, minimum (in dollars per loan)			150		150
Loan servicing costs - annually per loan based upon loan type, delinquent, maximum (in dollars per loan)			500		500
Loan servicing costs - annually per loan based upon loan type, foreclosure, minimum (in dollars per loan)			1,000		1,000
Loan servicing costs - annually per loan based upon loan type, foreclosure, maximum (in dollars per loan)			$ 4,000		$ 4,000
Escrow earnings rate - indexed to rates paid on deposit accounts with comparable average life (in hundredths)			0.34%		1.73%
Primary secondary mortgage rate spread	105		105		104
Delinquency rate			3.72%		2.73%