Mortgage Banking Activities, Mortgage Servicing Rights (Details)	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Summary of Mortgage Servicing Rights [Abstract]
Number of residential mortgage loans serviced for others	132,463,000	136,528,000	139,340,000
Outstanding principal balance of residential mortgage loans serviced for others	$ 21,658,335,000	$ 22,046,632,000	$ 21,997,568,000
Weighted average interest rate (in hundredths)	3.99%	3.94%	3.97%
Remaining contractual term (in months)	293 months	297 months	301 months
Servicing Asset at Fair Value, Amount [Roll Forward]
Beginning balance	$ 252,867,000	$ 247,073,000	$ 218,605,000
Additions, net	35,247,000	39,149,000	71,405,000
Change in fair value due to loan runoff	(33,528,000)	(33,527,000)	(40,744,000)
Change in fair value due to market changes	4,668,000	172,000	(2,193,000)
Ending balance	$ 259,254,000	$ 252,867,000	$ 247,073,000
Servicing Assets at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Discount rate - risk-free rate plus a market premium (in hundredths)	9.90%	9.84%
Prepayment rate - based upon loan interest rate, original term and loan type, minimum	8.05%	8.72%
Prepayment rate - based upon loan interest rate, original term and loan type, maximum	15.74%	15.16%
Loan servicing costs - annually per loan based upon loan type, performing, minimum (in dollars per loan)	$ 67	$ 65
Loan servicing costs - annually per loan based upon loan type, performing, maximum (in dollars per loan)	93	88
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
Primary secondary mortgage rate spread (in basis points)	105	105
Escrow earnings rate - indexed to rates paid on deposit accounts with comparable average life	2.57%	2.24%