Mortgage Banking Activities, Mortgage Servicing Rights (Details)	3 Months Ended				6 Months Ended
	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Summary of Mortgage Servicing Rights [Abstract]
Number of residential mortgage loans serviced for others	134,868		136,528	138,335	134,868	138,335
Outstanding principal balance of residential mortgage loans serviced for others	$ 21,963,309,000		$ 22,046,632,000	$ 22,095,232,000	$ 21,963,309,000	$ 22,095,232,000
Weighted average interest rate	3.96%		3.94%	3.95%	3.96%	3.95%
Remaining term (in months)	295 months	297 months		299 months
Servicing Asset at Fair Value, Amount [Roll Forward]
Beginning balance	$ 274,978,000	$ 252,867,000		$ 249,403,000	$ 252,867,000	$ 247,073,000
Additions, net	10,820,000			11,078,000	19,720,000	19,514,000
Change in fair value due to principal payments	(8,802,000)			(8,299,000)	(16,797,000)	(16,261,000)
Change in fair value due to market assumption changes	1,723,000			(6,943,000)	22,929,000	(5,087,000)
Ending balance	$ 278,719,000	$ 274,978,000	$ 252,867,000	$ 245,239,000	$ 278,719,000	$ 245,239,000
Servicing Assets at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Discount rate - risk-free rate plus a market premium (in hundredths)	9.91%		9.84%	9.84%
Prepayment rate - based upon loan interest rate, original term and loan type, minimum	8.12%		8.72%	8.61%
Prepayment rate - based upon loan interest rate, original term and loan type, maximum	15.08%		15.16%	15.91%
Loan servicing costs - annually per loan based upon loan type, performing, minimum (in dollars per loan)	$ 65		$ 65	$ 65
Loan servicing costs - annually per loan based upon loan type, performing, maximum (in dollars per loan)	88		88	120
Loan servicing costs - annually per loan based upon loan type, delinquent, minimum (in dollars per loan)	150		150	150
Loan servicing costs - annually per loan based upon loan type, delinquent, maximum (in dollars per loan)	500		500	500
Loan servicing costs - annually per loan based upon loan type, foreclosure, minimum (in dollars per loan)	1,000		1,000	1,000
Loan servicing costs - annually per loan based upon loan type, foreclosure, maximum (in dollars per loan)	$ 4,000		$ 4,000	$ 4,250
Escrow earnings rate - indexed to rates paid on deposit accounts with comparable average life (in hundredths)	2.88%		2.24%	1.95%
Primary secondary mortgage rate spread (in basis points)	105		105	105	105	105