Derivative Financial Instruments (Fair Value of Derivatives by Balance Sheet Location) (Details) - USD ($) $ in Thousands		Mar. 31, 2021	Dec. 31, 2020
Derivative Financial Instruments, Assets [Member]
Derivatives, Fair Value [Line Items]
Gross derivative asset positions		$ 54,200	$ 83,095
Amounts offset in balance sheet	[1]	3,495	67
Net amounts presented in balance sheet		50,705	83,028
Collateral pledged	[2]	0	0
Net amounts		50,705	83,028
Derivative Financial Instruments, Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross derivative liability positions		50,830	81,190
Amounts offset in balance sheet	[1]	3,495	67
Net amounts presented in balance sheet		47,335	81,123
Collateral pledged	[2]	39,945	74,698
Net amounts		7,390	6,425
Derivatives Designated as Cash Flow Hedging Instruments [Member] | Interest rate swaps [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets designated as a cash flow hedge		0	0
Derivatives Designated as Cash Flow Hedging Instruments [Member] | Interest rate swaps [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities designated as a cash flow hedge		1,569	1,958
Not Designated as Hedging Instrument [Member] | Interest rate swaps with customers [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets not designated		44,732	75,804
Not Designated as Hedging Instrument [Member] | Interest rate swaps with customers [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities not designated		3,589	68
Not Designated as Hedging Instrument [Member] | Mirror swaps with counterparties [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets not designated		3,497	67
Not Designated as Hedging Instrument [Member] | Mirror swaps with counterparties [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities not designated		44,903	76,248
Not Designated as Hedging Instrument [Member] | Risk participation agreements [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets not designated		2	22
Not Designated as Hedging Instrument [Member] | Risk participation agreements [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities not designated		1	2
Not Designated as Hedging Instrument [Member] | Interest rate lock commitments [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets not designated		2,776	7,202
Not Designated as Hedging Instrument [Member] | Interest rate lock commitments [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities not designated		102	0
Not Designated as Hedging Instrument [Member] | Forward sale commitments [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets not designated		3,193	0
Not Designated as Hedging Instrument [Member] | Forward sale commitments [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities not designated		$ 666	$ 2,914

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

[2] 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.