Stockholders' Equity and Dividends - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								7 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Apr. 30, 2007
Schedule Of Stockholders Equity [Line Items]
Preferred stock, shares authorized (in shares)	50,000,000.0				50,000,000.0				50,000,000.0	50,000,000.0	50,000,000.0
Preferred stock, par value (in dollars per share)	$ 0.01				$ 0.01				$ 0.01	$ 0.01	$ 0.01
Preferred stock, shares outstanding (in shares)	10,000,000.0				10,000,000.0				10,000,000.0	10,000,000.0	10,000,000.0
Common stock, shares authorized (in shares)	1,950,000,000				1,950,000,000				1,950,000,000	1,950,000,000	1,950,000,000
Common stock, par value (in dollars per share)	$ 0.01				$ 0.01				$ 0.01	$ 0.01	$ 0.01
Common stock, shares issued (in shares)	532,800,000				466,300,000				532,800,000	532,800,000	466,300,000
Number of common stock purchased (in shares)	89,200,000				89,000,000.0				89,200,000	89,200,000	89,000,000.0
Shares authorized to be repurchased (in shares)			20,000,000.0
Treasury stock shares acquired (in shares)									200,000
Treasury stock acquired									$ 2.5	$ 2.5
Original loan amount ESOP													$ 216.8
ESOP shares purchased in open market													10,500,000
Unallocated common stock of Employee Stock Ownership Plan (in shares)	5,923,692				6,300,000				5,923,692	5,923,692	6,300,000
Unallocated common stock of employee stock ownership plan	$ 122.9				$ 130.1				$ 122.9	$ 122.9	$ 130.1
Cash dividends on common stock (in dollars per share)										$ 0.7075	$ 0.6975	$ 0.6875
Dividend payout ratio	52.20%	53.10%	53.80%	58.60%	50.30%	52.90%	56.20%	56.30%		54.30%	53.70%	70.60%
Repurchases Authorized by Board of Directors [Member]
Schedule Of Stockholders Equity [Line Items]
Number of common stock purchased (in shares)	86,600,000								86,600,000	86,600,000
2007 Recognition and Retention Plan [Member]
Schedule Of Stockholders Equity [Line Items]
Number of common stock purchased (in shares)	2,600,000								2,600,000	2,600,000
People's United Financial, Inc. [Member]
Schedule Of Stockholders Equity [Line Items]
Preferred stock, shares authorized (in shares)	50,000,000.0								50,000,000.0	50,000,000.0
Preferred stock, par value (in dollars per share)	$ 0.01								$ 0.01	$ 0.01
Preferred stock, shares outstanding (in shares)	10,000,000.0								10,000,000.0	10,000,000.0
Common stock, shares authorized (in shares)	1,950,000,000								1,950,000,000	1,950,000,000
Common stock, par value (in dollars per share)	$ 0.01								$ 0.01	$ 0.01
Common stock, shares issued (in shares)	532,800,000								532,800,000	532,800,000
Cash dividends payable to parent company										$ 457.0	$ 342.0	$ 292.0
People's United Bank [Member]
Schedule Of Stockholders Equity [Line Items]
Retained net income under federal regulations										$ 233.3
2014 Long-Term Incentive Plan [Member] | 2007 Recognition and Retention Plan [Member]
Schedule Of Stockholders Equity [Line Items]
Additional awards under RRP	0								0	0