Amount, excluding accrued interest, of allowance for credit loss on investment in debt security measured at amortized cost (held-to-maturity).

Name:	us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLossExcludingAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityCreditQualityIndicatorLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount excluding accrued interest, after allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity).

+ References

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestAfterAllowanceForCreditLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount excluding accrued interest of investment in debt security measured at amortized cost (held-to-maturity) originated more than five years prior to current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919232-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 3B
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL120267963-210447

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount excluding accrued interest of investment in debt security measured at amortized cost (held-to-maturity) originated four years prior to current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 3B
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL120267963-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919232-210447

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount excluding accrued interest of investment in debt security measured at amortized cost (held-to-maturity) originated three years prior to current fiscal year.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount excluding accrued interest of investment in debt security measured at amortized cost (held-to-maturity) originated in current fiscal year.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestYearOneOriginatedCurrentFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount excluding accrued interest of investment in debt security measured at amortized cost (held-to-maturity) originated two years prior to current fiscal year.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount excluding accrued interest of investment in debt security measured at amortized cost (held-to-maturity) originated in fiscal year prior to current fiscal year.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919253-210447

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953401-111524

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27232-111563

+ Details

Name:	us-gaap_HeldToMaturitySecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=wtfc_InternalInvestmentGrade1To4Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=wtfc_InternalInvestmentGrade5To7Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=wtfc_InternalInvestmentGrade8To10Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MunicipalBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateNoteSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Allowance for Credit Losses (Held-to-Maturity Debt Securities by Credit Quality Indication) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	$ 2,736,805	$ 579,197	$ 560,330
Less: Allowance for credit losses	(83)	(59)	(63)
Held-to-maturity securities, net of allowance for credit losses	2,736,722	579,138	560,267
U.S. Government agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2021	97,790
2020	25,000
2019	0
2018	50,000
2017	0
Prior	3,353
Amortized Cost	176,143	177,959	349,415
U.S. Government agencies \| 1-4 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2021	97,790
2020	25,000
2019	0
2018	50,000
2017	0
Prior	3,353
Amortized Cost	176,143
U.S. Government agencies \| 5-7 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2021	0
2020	0
2019	0
2018	0
2017	0
Prior	0
Amortized Cost	0
U.S. Government agencies \| 8-10 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2021	0
2020	0
2019	0
2018	0
2017	0
Prior	0
Amortized Cost	0
Municipal
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2021	1,371
2020	0
2019	161
2018	7,508
2017	43,358
Prior	138,264
Amortized Cost	190,662	$ 200,707	$ 210,915
Municipal \| 1-4 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2021	1,371
2020	0
2019	161
2018	7,508
2017	43,358
Prior	138,264
Amortized Cost	190,662
Municipal \| 5-7 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2021	0
2020	0
2019	0
2018	0
2017	0
Prior	0
Amortized Cost	0
Municipal \| 8-10 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2021	0
2020	0
2019	0
2018	0
2017	0
Prior	0
Amortized Cost	0
Mortgage-backed securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2021	2,318,309
2020	0
2019	0
2018	0
2017	0
Prior	0
Amortized Cost	2,318,309
Mortgage-backed securities \| 1-4 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2021	2,318,309
2020	0
2019	0
2018	0
2017	0
Prior	0
Amortized Cost	2,318,309
Mortgage-backed securities \| 5-7 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2021	0
2020	0
2019	0
2018	0
2017	0
Prior	0
Amortized Cost	0
Mortgage-backed securities \| 8-10 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2021	0
2020	0
2019	0
2018	0
2017	0
Prior	0
Amortized Cost	0
Corporate notes
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2021	0
2020	13,578
2019	7,419
2018	3,285
2017	3,237
Prior	24,172
Amortized Cost	51,691
Corporate notes \| 1-4 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2021	0
2020	13,578
2019	7,419
2018	3,285
2017	3,237
Prior	24,172
Amortized Cost	51,691
Corporate notes \| 5-7 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2021	0
2020	0
2019	0
2018	0
2017	0
Prior	0
Amortized Cost	0
Corporate notes \| 8-10 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2021	0
2020	0
2019	0
2018	0
2017	0
Prior	0
Amortized Cost	$ 0