Mortgage Banking Activities, Mortgage Servicing Rights (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Summary of Mortgage Servicing Rights [Abstract]
Number of residential mortgage loans serviced for others	130,270		130,270		132,463
Outstanding principal balance of residential mortgage loans serviced for others	$ 21,349,914,000		$ 21,349,914,000		$ 21,658,335,000
Weighted average interest rate	4.01%		4.01%		3.99%
Remaining term (in months)			291 months		293 months
Servicing Asset at Fair Value, Amount [Roll Forward]
Beginning balance	$ 238,193,000	$ 274,978,000	$ 259,254,000	$ 252,867,000	$ 252,867,000
Additions, net	8,751,000	10,820,000	14,939,000	19,720,000
Change in fair value due to principal payments	(9,081,000)	(8,802,000)	(15,664,000)	(16,797,000)
Change in fair value due to market assumption changes	(29,555,000)	1,723,000	(50,221,000)	22,929,000
Ending balance	$ 208,308,000	$ 278,719,000	$ 208,308,000	$ 278,719,000	$ 259,254,000
Servicing Assets at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Discount rate - risk-free rate plus a market premium (in hundredths)			9.82%		9.90%
Prepayment rate - based upon loan interest rate, original term and loan type, minimum			8.28%		8.05%
Prepayment rate - based upon loan interest rate, original term and loan type, maximum			17.32%		15.74%
Loan servicing costs - annually per loan based upon loan type, performing, minimum (in dollars per loan)			$ 68		$ 67
Loan servicing costs - annually per loan based upon loan type, performing, maximum (in dollars per loan)			94		93
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)			1.75%		2.57%
Primary secondary mortgage rate spread	109		109		105