Derivative Financial Instruments (Schedule fair value of derivative instruments) (Detail) - USD ($) $ in Thousands		Jun. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivative instrument assets, Gross Amount		$ 6,517	$ 35,727
Asset Derivatives, Fair Value, Less: Cash collateral posted		30,700
Derivative Asset		1,035	0
Liability Derivatives, Fair Value, Gross Liability		4,987	25,300
Liability Derivatives, Fair Value, Less: Cash collateral posted		2,600
Derivative liability		186	446
Subject to and not subject to Master Netting Agreements [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		4,357,306	4,104,713
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement		48,659	77,637
Asset Derivatives, Fair Value, Less: Legally enforceable master netting agreements		3,802	24,252
Asset Derivatives, Fair Value, Less: Cash collateral posted		1,680	11,475
Derivative Asset		43,177	41,910
Liability Derivatives, Notional Amount		2,995,879	2,956,313
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		21,445	45,189
Liability Derivatives, Fair Value, Less: Legally enforceable master netting agreements		3,802	24,254
Liability Derivatives, Fair Value, Less: Cash collateral posted		999	600
Derivative liability		16,644	20,335
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative instrument assets, Gross Amount		2,198	3,270
Asset Derivatives, Fair Value, Less: Legally enforceable master netting agreements		1,046	2,335
Asset Derivatives, Fair Value, Less: Cash collateral posted		398	935
Derivative Asset		754	0
Liability Derivatives, Fair Value, Gross Liability		367	792
Liability Derivatives, Fair Value, Less: Legally enforceable master netting agreements		367	792
Liability Derivatives, Fair Value, Less: Cash collateral posted		0	0
Derivative liability		0	0
Designated as Hedging Instrument [Member] | Derivatives Subject to Master Netting Arrangement or Similar Arrangement [Member] | Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts	[1]	225,000	225,000
Derivative instrument assets, Gross Amount	[1]	2,198	3,270
Liability Derivatives, Notional Amount	[1]	100,000	100,000
Liability Derivatives, Fair Value, Gross Liability	[1]	367	792
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative instrument assets, Gross Amount		4,319	32,457
Asset Derivatives, Fair Value, Less: Legally enforceable master netting agreements		2,756	21,917
Asset Derivatives, Fair Value, Less: Cash collateral posted		1,282	10,540
Derivative Asset		281	0
Liability Derivatives, Fair Value, Gross Liability		4,620	24,508
Liability Derivatives, Fair Value, Less: Legally enforceable master netting agreements		3,435	23,462
Liability Derivatives, Fair Value, Less: Cash collateral posted		999	600
Derivative liability		186	446
Not Designated as Hedging Instrument [Member] | Derivatives Subject to Master Netting Arrangement or Similar Arrangement [Member] | Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts	[1]	2,132,888	1,943,485
Derivative instrument assets, Gross Amount	[1]	6,740	32,226
Liability Derivatives, Notional Amount	[1]	1,205,406	1,242,937
Liability Derivatives, Fair Value, Gross Liability	[1]	4,224	24,388
Not Designated as Hedging Instrument [Member] | Derivatives Subject to Master Netting Arrangement or Similar Arrangement [Member] | Other [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts	[1]	3,566	10,634
Derivative instrument assets, Gross Amount	[1]	109	231
Liability Derivatives, Notional Amount	[1]	22,588	14,265
Liability Derivatives, Fair Value, Gross Liability	[1]	395	120
Not Designated as Hedging Instrument [Member] | Derivatives Not Subject to Master Netting Arrangement or Similar Arrangement [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		4,132,306	3,879,713
Asset Derivatives, Fair Value, Positions not subject to master netting agreement		46,461	74,367
Liability Derivatives, Notional Amount		2,895,879	2,856,313
Liability Derivatives, Fair Value, Positions not subject to master netting agreement		21,078	44,397
Not Designated as Hedging Instrument [Member] | Derivatives Not Subject to Master Netting Arrangement or Similar Arrangement [Member] | Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		1,822,285	1,734,679
Asset Derivatives, Fair Value, Positions not subject to master netting agreement		38,495	38,668
Liability Derivatives, Notional Amount		1,516,009	1,451,762
Liability Derivatives, Fair Value, Positions not subject to master netting agreement		15,915	19,001
Not Designated as Hedging Instrument [Member] | Derivatives Not Subject to Master Netting Arrangement or Similar Arrangement [Member] | RPAs [Member] | RPA-Out [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		85,447	86,037
Asset Derivatives, Fair Value, Positions not subject to master netting agreement		119	139
Not Designated as Hedging Instrument [Member] | Derivatives Not Subject to Master Netting Arrangement or Similar Arrangement [Member] | RPAs [Member] | RPA-In [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives, Notional Amount		97,528	87,273
Liability Derivatives, Fair Value, Positions not subject to master netting agreement		145	166
Not Designated as Hedging Instrument [Member] | Derivatives Not Subject to Master Netting Arrangement or Similar Arrangement [Member] | Mortgage Banking Derivatives [Member] | RPA-Out [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		88,120	103,440
Asset Derivatives, Fair Value, Positions not subject to master netting agreement		998	3,084
Not Designated as Hedging Instrument [Member] | Derivatives Not Subject to Master Netting Arrangement or Similar Arrangement [Member] | Mortgage Banking Derivatives [Member] | RPA-In [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives, Notional Amount		52,491	59,895
Liability Derivatives, Fair Value, Positions not subject to master netting agreement		293	711
Not Designated as Hedging Instrument [Member] | Derivatives Not Subject to Master Netting Arrangement or Similar Arrangement [Member] | Other [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		0	1,438
Asset Derivatives, Fair Value, Positions not subject to master netting agreement		0	19
Liability Derivatives, Notional Amount		1,857	181
Liability Derivatives, Fair Value, Positions not subject to master netting agreement		106	$ 11
Loan Origination Commitments [Member]
Derivatives, Fair Value [Line Items]
Other Commitment		77,900
Interest Rate Lock Commitments [Member]
Derivatives, Fair Value [Line Items]
Other Commitment		$ 22,800

[1] One of Webster's counterparty relationships was impacted by a Chicago Mercantile Exchange rulebook amendment, resulting in the presentation of that relationship on a settlement basis, as a single unit of account at June 30, 2017, versus a netting basis at December 31, 2016.