Regulatory Capital Requirements - Regulatory Capital Requirements and Ratio under Basel III Capital Rules (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Leverage Capital, Ratio	4.00%
Total, Ratio	8.00%
CET 1 Risk-Based Capital, Ratio, Minimum Capital Required	4.50%
Tier 1 Risk-Based Capital, Ratio, Minimum Capital Required	6.00%
Total Risk-Weighted Assets for allowance for loan losses	1.25%
People's United Financial, Inc. [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Leverage Capital, Amount	$ 4,846.1	$ 3,927.2
CET 1 Risk-Based Capital, Amount	4,602.1	3,683.1
Tier 1 Risk-Based Capital	4,846.1	3,927.2
Total, Amount	$ 5,435.6	$ 4,505.7
Tier 1 Leverage Capital, Ratio	9.10%	8.70%
CET 1 Risk-Based Capital, Ratio	10.20%	10.30%
Tier 1 Risk-Based Capital, Ratio	10.70%	10.90%
Total, Ratio	12.00%	12.50%
Tier 1 Leverage Capital, Minimum Capital Required, Amount	$ 2,119.7	$ 1,806.0
CET 1 Risk-Based Capital, Minimum Capital Required, Amount	3,164.5	2,289.3
Tier 1 Risk-Based Capital, Minimum Capital Required, Amount	3,842.6	2,827.9
Total, Minimum Capital Required, Amount	$ 4,746.8	$ 3,546.1
Tier 1 Leverage Capital, Ratio, Minimum Capital Required	4.00%	4.00%
CET 1 Risk-Based Capital, Ratio, Minimum Capital Required	7.00%	6.375%
Tier 1 Risk-Based Capital, Ratio, Minimum Capital Required	8.50%	7.875%
Total, Ratio, Minimum Capital Required	10.50%	9.875%
Tier 1 Risk-Based Capital, Well-Capitalized, Amount	$ 2,712.5	$ 2,154.6
Total, Well-Capitalized, Amount	$ 4,520.8	$ 3,591.0
Tier 1 Risk-Based Capital, Ratio, Well-Capitalized	6.00%	6.00%
Total, Ratio, Well-Capitalized	10.00%	10.00%
Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Leverage Capital, Amount	$ 4,902.0	$ 4,076.0
CET 1 Risk-Based Capital, Amount	4,902.0	4,076.0
Tier 1 Risk-Based Capital	4,902.0	4,076.0
Total, Amount	$ 5,474.2	$ 4,719.1
Tier 1 Leverage Capital, Ratio	9.30%	9.00%
CET 1 Risk-Based Capital, Ratio	10.90%	11.40%
Tier 1 Risk-Based Capital, Ratio	10.90%	11.40%
Total, Ratio	12.10%	13.20%
Tier 1 Leverage Capital, Minimum Capital Required, Amount	$ 2,119.1	$ 1,805.4
CET 1 Risk-Based Capital, Minimum Capital Required, Amount	3,162.2	2,287.1
Tier 1 Risk-Based Capital, Minimum Capital Required, Amount	3,839.8	2,825.2
Total, Minimum Capital Required, Amount	$ 4,743.3	$ 3,542.7
Tier 1 Leverage Capital, Ratio, Minimum Capital Required	4.00%	4.00%
CET 1 Risk-Based Capital, Ratio, Minimum Capital Required	7.00%	6.375%
Tier 1 Risk-Based Capital, Ratio, Minimum Capital Required	8.50%	7.875%
Total, Ratio, Minimum Capital Required	10.50%	9.875%
Tier 1 Leverage Capital, Well-Capitalized, Amount	$ 2,648.8	$ 2,256.8
CET 1 Risk-Based Capital, Well-Capitalized, Amount	2,936.3	2,331.9
Tier 1 Risk-Based Capital, Well-Capitalized, Amount	3,614.0	2,870.0
Total, Well-Capitalized, Amount	$ 4,517.4	$ 3,587.5
Tier 1 Leverage Capital, Ratio , Well-Capitalized	5.00%	5.00%
CET 1 Risk-Based Capital, Ratio, Well-Capitalized	6.50%	6.50%
Tier 1 Risk-Based Capital, Ratio, Well-Capitalized	8.00%	8.00%
Total, Ratio, Well-Capitalized	10.00%	10.00%