Derivative Financial Instruments (Fair Value of Derivatives by Balance Sheet Location) (Details) - USD ($) $ in Thousands		Sep. 30, 2023	Dec. 31, 2022
Derivatives, Fair Value [Line Items]
Accrued interest receivable		$ 22,000	$ 17,600
Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Notional amount		140,000	320,000
Interest rate swaps [Member] | Designated as Cash Flow Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Notional amount		140,000	320,000
Interest rate swaps [Member] | Designated as Cash Flow Hedging Instruments [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Interest rate cash flow hedge asset at fair value	[1]	3,639	548
Accrued interest receivable		262	24
Interest rate swaps [Member] | Designated as Cash Flow Hedging Instruments [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Interest rate cash flow hedge liability at fair value	[1]	0	31,178
Accrued interest payable			856
Interest rate contracts with customers [Member]
Derivatives, Fair Value [Line Items]
Notional amount		962,174	935,099
Interest rate contracts with customers [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Notional amount		962,174	935,099
Interest rate contracts with customers [Member] | Not Designated as Hedging Instrument [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets not designated		1,839	32
Interest rate contracts with customers [Member] | Not Designated as Hedging Instrument [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities not designated		80,658	68,137
Mirror interest rate contracts with counterparties [Member]
Derivatives, Fair Value [Line Items]
Notional amount		962,174	935,099
Mirror interest rate contracts with counterparties [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Notional amount		962,174	935,099
Mirror interest rate contracts with counterparties [Member] | Not Designated as Hedging Instrument [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets not designated		80,277	67,797
Mirror interest rate contracts with counterparties [Member] | Not Designated as Hedging Instrument [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities not designated		1,841	61
Risk participation agreements [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Notional amount		319,002	282,191
Risk participation agreements [Member] | Not Designated as Hedging Instrument [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets not designated		1	0
Risk participation agreements [Member] | Not Designated as Hedging Instrument [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities not designated		0	2
Interest rate lock commitments [Member]
Derivatives, Fair Value [Line Items]
Notional amount		30,722	12,201
Interest rate lock commitments [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Notional amount		30,722	12,201
Interest rate lock commitments [Member] | Not Designated as Hedging Instrument [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets not designated		334	144
Interest rate lock commitments [Member] | Not Designated as Hedging Instrument [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities not designated		20	4
Forward sale commitments [Member]
Derivatives, Fair Value [Line Items]
Notional amount		45,433	23,150
Forward sale commitments [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Notional amount		45,433	23,150
Forward sale commitments [Member] | Not Designated as Hedging Instrument [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets not designated		192	58
Forward sale commitments [Member] | Not Designated as Hedging Instrument [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities not designated		139	150
Derivative assets[Member]
Derivatives, Fair Value [Line Items]
Gross derivative asset positions		86,282	68,579
Less: derivative asset amounts offset	[2]	1,841	23,524
Derivative asset balances, net of offset		84,441	45,055
Less: collateral pledged	[3]	0	0
Net amounts		84,441	45,055
Derivative liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross derivative liability positions		82,658	99,532
Less: derivative liability amounts offset	[2]	1,841	23,524
Derivative liability balances, net of offset		80,817	76,008
Less: collateral pledged	[3]	0	7,716
Net amounts		$ 80,817	$ 68,292

[1] The fair value of derivative assets includes accrued interest receivable of $262 thousand and $24 thousand, respectively, at September 30, 2023 and December 31, 2022. The fair value of derivative liabilities includes accrued interest payable of $856 thousand at December 31, 2022.

[2] Interest rate risk management contracts and loan related derivative contracts with counterparties are subject to master netting arrangements.

[3] Collateral pledged to derivative counterparties is in the form of cash. Washington Trust may need to post additional collateral in the future in proportion to potential increases in unrealized loss positions.