Employee Benefit Plans - People's United Employee Pension and Other Postretirement Plans - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Feb. 28, 2018
Defined Benefit Plan Disclosure [Line Items]
Average compensation term on which pension plan benefits are based, in years			5 years
Average compensation term within last ten years on which pension plan benefits are based, in years			5 years
Term considered when determining employee pension benefits, in years			10 years
Trust assets	$ 56,400,000		$ 51,000,000.0	$ 56,400,000
Maximum percentage of fixed income holdings invested in issues rated below Baa			30.00%
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of covered employee's eligible compensation			4.00%
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	596,400,000		$ 746,700,000	596,400,000	$ 584,900,000
Projected benefit obligations:	562,700,000		665,500,000	562,700,000	$ 584,800,000
Defined benefit plan, expected future benefit payments in year one			28,500,000
Defined benefit plan, expected future benefit payments in year two			30,000,000.0
Defined benefit plan, expected future benefit payments in year three			30,600,000
Defined benefit plan, expected future benefit payments in year four			31,200,000
Defined benefit plan, expected future benefit payments in year five			33,000,000.0
Defined benefit plan, expected future benefit payments after five years			181,200,000
Pension Plans [Member] | Nonqualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations:	44,000,000.0		40,200,000	44,000,000.0
Trust assets			33,100,000
Pension Plans [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	596,400,000		746,700,000	596,400,000
Projected benefit obligations:	$ 518,700,000		625,300,000	$ 518,700,000
Net periodic benefit income			$ 22,300,000
Percentage of equity holdings invested in international equities			50.00%
Maximum percentage of fixed income holdings invested in issues rated below Baa			2.50%
Market value of total plan assets			2.50%
Maximum percentage of any individual security			3.00%
Pension Plans [Member] | Qualified Plan [Member] | People's Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of covered employee's eligible compensation			3.00%
Minimum age requirement to participate in pension plan, years			18 years
Minimum service hours per year requirement to participate in pension plan			1000 hours
Pension plan assets measured at fair value			$ 687,800,000
Projected benefit obligations:			$ 569,100,000
Amortization of unrecognized gain loss over average remaining life of plan participants			26 years
Discount rate used to determine benefit obligation	4.41%		3.38%	4.41%	3.74%
Discount rate used to calculate net periodic benefit cost	4.34%	3.74%	4.41%		4.41%
Expected long-term rate of return assumption			7.25%	7.25%	7.25%
Expected long-term rate of return assumption			7.25%
Discount rate			3.38%
Expected employer contributions						$ 40,000,000
Pension Plans [Member] | Qualified Plan [Member] | United Financial Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value			$ 34,000,000.0
Projected benefit obligations:			$ 31,900,000
Amortization of unrecognized gain loss over average remaining life of plan participants			30 years
Discount rate used to determine benefit obligation			3.42%
Discount rate used to calculate net periodic benefit cost			3.33%
Expected long-term rate of return assumption			5.25%
Discount rate			3.42%
Pension Plans [Member] | Qualified Plan [Member] | First Connecticut Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value			$ 24,900,000
Projected benefit obligations:			$ 24,300,000
Amortization of unrecognized gain loss over average remaining life of plan participants			28 years
Discount rate used to determine benefit obligation	4.41%		3.39%	4.41%
Discount rate used to calculate net periodic benefit cost			4.41%	4.35%
Expected long-term rate of return assumption			6.00%	6.00%
Expected long-term rate of return assumption			7.25%
Discount rate			3.39%
Pension Plans [Member] | Qualified Plan [Member] | Chittenden Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate used to determine benefit obligation					3.62%
Discount rate used to calculate net periodic benefit cost					4.16%
Expected employer contributions						10,000,000
Pension Plans [Member] | Qualified Plan [Member] | Suffolk Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate used to determine benefit obligation					3.72%
Discount rate used to calculate net periodic benefit cost					4.24%
Expected employer contributions						$ 0
Pension Plans [Member] | Minimum [Member] | Nonqualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate used to determine benefit obligation	4.00%		2.76%	4.00%
Discount rate used to calculate net periodic benefit cost			3.33%	3.32%
Pension Plans [Member] | Minimum [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated unrecognized gain or loss percentage for amortization of actuarial gain loss			10.00%
Pension Plans [Member] | Maximum [Member] | Nonqualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate used to determine benefit obligation	4.40%		3.40%	4.40%
Discount rate used to calculate net periodic benefit cost			4.40%	4.30%
Other Pension Plan, Postretirement or Supplemental Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions			$ 4,900,000
Other Postretirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	$ 0		0	$ 0	$ 0
Projected benefit obligations:	$ 15,300,000		18,700,000	$ 15,300,000	$ 14,600,000
Defined benefit plan, expected future benefit payments in year one			1,100,000
Defined benefit plan, expected future benefit payments in year two			1,100,000
Defined benefit plan, expected future benefit payments in year three			1,100,000
Defined benefit plan, expected future benefit payments in year four			1,100,000
Defined benefit plan, expected future benefit payments in year five			1,000,000.0
Defined benefit plan, expected future benefit payments after five years			$ 4,700,000
Other Postretirement Plans [Member] | People's Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate used to determine benefit obligation	4.40%		3.40%	4.40%	3.70%
Discount rate used to calculate net periodic benefit cost			4.40%	3.70%	4.40%