Regulatory Capital Requirements - Additional Information (Detail) - USD ($)		12 Months Ended
	Jan. 01, 2019	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2020
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier 1 capital, Ratio, Minimum capital adequacy		4.50%
Capital conservation buffer		2.50%
Common Equity Tier 1 Capital, Ratio, Minimum Capital Adequacy including conservation buffer		7.00%
Risk-based capital tier 1, Ratio, Minimum capital adequacy		6.00%
Common Tier 1 buffer phase start year		2016
Risk-based capital tier 1, Ratio, Minimum capital adequacy including conservation buffer		8.50%
Total, Ratio, Minimum capital adequacy		8.00%
Total, Ratio, Minimum capital adequacy including conservation buffer		10.50%
Leverage (core) capital, Ratio		4.00%
Capital conservation buffer		2.50%	1.875%
Operating lease		$ 307,900,000
Lease ROU assets		$ 276,600,000
Accounting Standards Update 2016-02 [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Operating lease	$ 268,800,000
Lease ROU assets	$ 248,500,000
Increase (decrease) in risk-based capital ratios	(0.10%)
Minimum [Member] | Accounting Standards Update 2016-02 [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Increase (decrease) in risk-based capital ratios	(0.05%)
Minimum [Member] | Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member] | Scenario, Forecast [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Decrease in risk-based capital ratios				(0.10%)
Maximum [Member] | Accounting Standards Update 2016-02 [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Increase (decrease) in risk-based capital ratios	(0.10%)
People's United Financial, Inc. [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier 1 capital, Ratio, Minimum capital adequacy		7.00%	6.375%
Risk-based capital tier 1, Ratio, Minimum capital adequacy		8.50%	7.875%
Total, Ratio, Minimum capital adequacy		12.00%	12.50%
Leverage (core) capital, Ratio		9.10%	8.70%
Risk-weighted assets		$ 45,200,000,000	$ 35,900,000,000
People's United Financial, Inc. [Member] | 4.00% Subordinated Notes Due 2024 [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Reduction period		4 years
Senior notes issuance		$ 75,000,000
Reduction percentage		20.00%
Reduction amount		$ 15,000,000
Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier 1 capital, Ratio, Minimum capital adequacy		7.00%	6.375%
Risk-based capital tier 1, Ratio, Minimum capital adequacy		8.50%	7.875%
Total, Ratio, Minimum capital adequacy		12.10%	13.20%
Leverage (core) capital, Ratio		9.30%	9.00%
Bank [Member] | 4.00% Subordinated Notes Due 2024 [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Senior notes issuance		$ 400,000,000
Reduction percentage		20.00%
Reduction amount		$ 80,000,000