Derivative Financial Instruments (Narrative) (Details) $ in Thousands	Dec. 31, 2018 USD ($) derivative_instrument	Dec. 31, 2017 USD ($) derivative_instrument
Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Notional amount	$ 60,000	$ 60,000
Interest rate swaps with customers [Member]
Derivatives, Fair Value [Line Items]
Notional amount	648,050	545,049
Mirror swaps with counterparties [Member]
Derivatives, Fair Value [Line Items]
Notional amount	648,050	545,049
Risk participation-in agreement [Member]
Derivatives, Fair Value [Line Items]
Notional amount	46,510	34,052
Foreign exchange contracts [Member]
Derivatives, Fair Value [Line Items]
Notional amount	2,784	3,005
Interest rate lock commitments [Member]
Derivatives, Fair Value [Line Items]
Notional amount	30,766	45,139
Commitments to sell mortgage loans [Member]
Derivatives, Fair Value [Line Items]
Notional amount	$ 61,993	71,539
Designated as Hedging Instrument [Member] \| Interest rate caps [Member]
Derivatives, Fair Value [Line Items]
Number of derivative instruments held \| derivative_instrument	2
Notional amount	$ 22,700	$ 22,700
Interest rate cap strike	4.50%
Designated as Hedging Instrument [Member] \| Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Number of derivative instruments held \| derivative_instrument	2	2
Notional amount	$ 60,000	$ 60,000
Designated as Hedging Instrument [Member] \| Interest rate floors [Member]
Derivatives, Fair Value [Line Items]
Number of derivative instruments held \| derivative_instrument	3	3
Notional amount	$ 300,000	$ 300,000
Interest rate floor strike	1.00%
Not Designated as Hedging Instrument [Member] \| Interest rate swaps with customers [Member]
Derivatives, Fair Value [Line Items]
Notional amount	$ 648,000	545,000
Not Designated as Hedging Instrument [Member] \| Risk participation-out agreement [Member]
Derivatives, Fair Value [Line Items]
Notional amount	57,300	52,400
Not Designated as Hedging Instrument [Member] \| Risk participation-in agreement [Member]
Derivatives, Fair Value [Line Items]
Notional amount	46,500	34,100
Not Designated as Hedging Instrument [Member] \| Foreign exchange contracts [Member]
Derivatives, Fair Value [Line Items]
Notional amount	$ 2,784	$ 3,005

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Cap rate on an interest rate derivative such as an interest rate cap or collar. If market rates exceed the cap rate, a payment or receipt is triggered on the contract.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeCapInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Floor rate on an interest rate derivative such as an interest rate floor or collar. If market rates falls below the floor rate, a payment or receipt is triggered on the contract.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeFloorInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of derivative instruments of a particular group held by the entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

+ Details

Name:	us-gaap_DerivativeNumberOfInstrumentsHeld
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
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

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
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_RiskparticipationinagreementMember
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:

- 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_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_RiskparticipationoutagreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: