Mortgage Banking Activities, Mortgage Servicing Rights (Details)	3 Months Ended	12 Months Ended
Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Summary of Mortgage Servicing Rights [Abstract]
Number of residential mortgage loans serviced for others	131,636,000		132,463,000
Outstanding principal balance of residential mortgage loans serviced for others	$ 21,544,295,000		$ 21,658,335,000
Weighted average interest rate	4.00%		3.99%
Remaining term (in months)	292 months		293 months
Servicing Asset at Fair Value, Amount [Roll Forward]
Beginning balance	$ 259,254,000	$ 252,867,000	$ 252,867,000
Additions, net	6,188,000	8,900,000
Change in fair value due to principal payments	(6,583,000)	(7,995,000)
Change in fair value due to market assumption changes	(20,666,000)	21,206,000
Ending balance	$ 238,193,000	$ 274,978,000	$ 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)	9.83%		9.90%
Prepayment rate - based upon loan interest rate, original term and loan type, minimum	8.14%		8.05%
Prepayment rate - based upon loan interest rate, original term and loan type, maximum	16.02%		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)	2.29%		2.57%
Primary secondary mortgage rate spread	105		105

The decrease in fair value as a result of loan runoff.

No definition available.

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- Definition

Mortgage rate spread between the primary and secondary market.

+ References

No definition available.

+ Details

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- Definition

Indicates the number of mortgage loans being serviced.

+ References

No definition available.

+ Details

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- Definition

Dollar amount of principal outstanding on mortgage loans being serviced.

+ References

No definition available.

+ Details

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- Definition

Represents the weighted average remaining term of mortgage loans being serviced fro others.

+ References

No definition available.

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- Definition

Servicing Assets at Fair Value Assumptions Used to Estimate Fair Value Delinquent Loan Service Cost Minimum

+ References

No definition available.

+ Details

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- Definition

Servicing Assets At Fair Value Assumptions Used to Estimate Fair Value Delinquent

+ References

No definition available.

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- Definition

The escrow earning rate used to estimate the fair value of servicing assets, based on rates paid on deposit accounts.

+ References

No definition available.

+ Details

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- Definition

Servicing Assets At Fair Value Assumptions Used to Estimate Fair Value Foreclosure Loan Service Cost Maximum

+ References

No definition available.

+ Details

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- Definition

Servicing Assets at Fair Value Assumptions Used to Estimate Fair Value Foreclosure Loan Service Cost Minimum

+ References

No definition available.

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- Definition

The maximum cost to service a performing loan for the period used as a variable in estimating fair value.

+ References

No definition available.

+ Details

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- Definition

The minimum cost to service a performing loan for the period used as a variable in estimating fair value.

+ References

No definition available.

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- Definition

The maximum estimated speed of prepayments of principal on financial assets which may be one of the factors applied in estimating the fair value of servicing assets.

+ References

No definition available.

+ Details

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- Definition

The minimum estimated speed of prepayments of principal on financial assets which may be one of the factors applied in estimating the fair value of servicing assets.

+ References

No definition available.

+ Details

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- Definition

-- None. No documentation exists for this element. --

+ References

No definition available.

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- Definition

Represents the weighted average interest rate of loans servicedf for others as of the balance sheet date.

+ References

No definition available.

+ Details

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- Definition

Amount of increase (decrease) in fair value from changes in the assumptions or model used to calculate the fair value of a contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

+ References

No definition available.

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- Definition

The value of new servicing assets, subsequently measured at fair value, acquired or created during the current period through purchases or from transfers of financial assets.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746

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- Definition

Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 50
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-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Discount rate used to estimate the fair value of servicing assets and servicing liabilities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122596-111746

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