Investment Securities (Marketable Securities) (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, amortized cost	$ 2,185,494,000	$ 1,833,069,000
Available-for-sale securities, Gross unrealized gains	6,780,000	6,624,000
Available-for-sale securities, Gross unrealized losses	(66,193,000)	(36,027,000)
Available-for-sale securities, fair value	2,126,081,000	1,803,666,000
Held-to-maturity securities	1,067,439,000	826,449,000
Held-to-maturity securities, Gross unrealized gains	2,241,000	2,691,000
Held-to-maturity securities, Gross unrealized losses	(33,584,000)	(16,624,000)
Held-to-maturity securities, fair value	1,036,096,000	812,516,000
Equity securities, FV-NI, cost	34,410,000	0
Equity securities, FV-NI, accumulated unrealized gain	1,532,000	0
Equity securities, FV-NI, accumulated unrealized loss	(1,225,000)	0
Equity securities with readily determinable fair value	34,717,000	0
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, amortized cost	126,199,000	144,904,000
Available-for-sale securities, Gross unrealized gains	391,000	0
Available-for-sale securities, Gross unrealized losses	(186,000)	(1,082,000)
Available-for-sale securities, fair value	126,404,000	143,822,000
U.S. Government agencies
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, amortized cost	139,420,000	157,638,000
Available-for-sale securities, Gross unrealized gains	917,000	2,000
Available-for-sale securities, Gross unrealized losses	(30,000)	(725,000)
Available-for-sale securities, fair value	140,307,000	156,915,000
Held-to-maturity securities	814,864,000	579,062,000
Held-to-maturity securities, Gross unrealized gains	1,141,000	23,000
Held-to-maturity securities, Gross unrealized losses	(28,576,000)	(14,066,000)
Held-to-maturity securities, fair value	787,429,000	565,019,000
Municipal
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, amortized cost	136,831,000	113,197,000
Available-for-sale securities, Gross unrealized gains	2,427,000	2,712,000
Available-for-sale securities, Gross unrealized losses	(768,000)	(557,000)
Available-for-sale securities, fair value	138,490,000	115,352,000
Held-to-maturity securities	252,575,000	247,387,000
Held-to-maturity securities, Gross unrealized gains	1,100,000	2,668,000
Held-to-maturity securities, Gross unrealized losses	(5,008,000)	(2,558,000)
Held-to-maturity securities, fair value	248,667,000	247,497,000
Corporate notes, financial issuers
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, amortized cost	97,079,000	30,309,000
Available-for-sale securities, Gross unrealized gains	35,000	43,000
Available-for-sale securities, Gross unrealized losses	(7,069,000)	(301,000)
Available-for-sale securities, fair value	90,045,000	30,051,000
Corporate notes, other
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, amortized cost	1,000,000	1,000,000
Available-for-sale securities, Gross unrealized gains	0	0
Available-for-sale securities, Gross unrealized losses	0	(1,000)
Available-for-sale securities, fair value	1,000,000	999,000
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, amortized cost	1,641,146,000	1,291,695,000
Available-for-sale securities, Gross unrealized gains	2,510,000	446,000
Available-for-sale securities, Gross unrealized losses	(57,317,000)	(31,955,000)
Available-for-sale securities, fair value	1,586,339,000	1,260,186,000
Collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, amortized cost	43,819,000	60,092,000
Available-for-sale securities, Gross unrealized gains	500,000	64,000
Available-for-sale securities, Gross unrealized losses	(823,000)	(617,000)
Available-for-sale securities, fair value	43,496,000	59,539,000
Equity securities with readily determinable fair value
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, amortized cost	0	34,234,000
Available-for-sale securities, Gross unrealized gains	0	3,357,000
Available-for-sale securities, Gross unrealized losses	0	(789,000)
Available-for-sale securities, fair value	0	$ 36,802,000
Subprime
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, fair value	$ 0

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

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

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

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

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

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

- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

+ Details

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

- Definition

Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

+ References

No definition available.

+ Details

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

- Definition

Amount of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: 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=82897076&loc=d3e27232-111563

+ Details

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

- Definition

Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

+ Details

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

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

+ Details

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

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Equity Securities, FV-NI, Accumulated Unrealized Gain

+ References

No definition available.

+ Details

Name:	wtfc_EquitySecuritiesFVNIAccumulatedUnrealizedGain
Namespace Prefix:	wtfc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Equity Securities, FV-NI, Accumulated Unrealized Loss

+ References

No definition available.

+ Details

Name:	wtfc_EquitySecuritiesFVNIAccumulatedUnrealizedLoss
Namespace Prefix:	wtfc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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