Derivative Financial Instruments (Fair Value of Derivatives by Balance Sheet Location) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Total derivative assets	$ 17,136	$ 2,759
Total derivative liabilities	14,933	3,047
Other assets [Member]
Derivatives, Fair Value [Line Items]
Total derivative assets	17,136	2,759
Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Total derivative liabilities	14,933	3,047
Derivatives Designated as Cash Flow Hedging Instruments [Member] \| Interest rate caps [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset designated as a cash flow hedge	71	25
Derivatives Designated as Cash Flow Hedging Instruments [Member] \| Interest rate caps [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability designated as a cash flow hedge	0	0
Derivatives Designated as Cash Flow Hedging Instruments [Member] \| Interest rate swaps [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset designated as a cash flow hedge	1,699	213
Derivatives Designated as Cash Flow Hedging Instruments [Member] \| Interest rate swaps [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability designated as a cash flow hedge	0	14
Derivatives Designated as Cash Flow Hedging Instruments [Member] \| Interest rate floors [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset designated as a cash flow hedge	125	110
Derivatives Designated as Cash Flow Hedging Instruments [Member] \| Interest rate floors [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability designated as a cash flow hedge	0	0
Not Designated as Hedging Instrument [Member] \| Interest rate swaps with customers [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset not designated	60	268
Not Designated as Hedging Instrument [Member] \| Interest rate swaps with customers [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability not designated	14,229	1,295
Not Designated as Hedging Instrument [Member] \| Mirror swaps with counterparties [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset not designated	14,037	1,152
Not Designated as Hedging Instrument [Member] \| Mirror swaps with counterparties [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability not designated	60	268
Not Designated as Hedging Instrument [Member] \| Risk participation agreements [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset not designated	0	0
Not Designated as Hedging Instrument [Member] \| Risk participation agreements [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability not designated	0	0
Not Designated as Hedging Instrument [Member] \| Foreign exchange contracts [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset not designated	0	0
Not Designated as Hedging Instrument [Member] \| Foreign exchange contracts [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability not designated	21	26
Not Designated as Hedging Instrument [Member] \| Interest rate lock commitments [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset not designated	1,130	965
Not Designated as Hedging Instrument [Member] \| Interest rate lock commitments [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability not designated	9	20
Not Designated as Hedging Instrument [Member] \| Forward sale commitments [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset not designated	14	26
Not Designated as Hedging Instrument [Member] \| Forward sale commitments [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability not designated	$ 614	$ 1,424

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313

+ Details

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

- Definition

Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as an asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959

+ Details

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

- Definition

Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as a liability.

+ References

+ Details

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

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

+ Details

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

- Definition

Fair value as of the balance sheet date of all interest rate derivative assets designated as cash flow hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959

+ Details

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

- Definition

Fair value as of the balance sheet date of all interest rate derivative liabilities designated as cash flow hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959

+ Details

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

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateCapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateFloorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=wash_InterestRateSwapwithcustomersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=wash_InterestRateMirrorSwapsWithCounterpartiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=wash_RiskparticipationagreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateLockCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForwardContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: