Parent Company Financial Information - Condensed Statements of Comprehensive Income (Detail) - USD ($) $ in Millions	3 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, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Net income	$ 137.5	$ 135.1	$ 133.2	$ 114.6	$ 132.9	$ 117.0	$ 110.2	$ 107.9	$ 520.4	$ 468.1	$ 337.2
Other comprehensive (loss) income, net of tax:
Total other comprehensive income (loss), net of tax									89.9	(37.8)	13.3
Total comprehensive income									610.3	430.3	350.5
People's United Financial, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									520.4	468.1	337.2
Other comprehensive (loss) income, net of tax:
Net unrealized losses on securities available-for-sale									0.0	0.0	(0.1)
Net unrealized losses on derivatives accounted for as cash flow hedges									(0.1)
Net unrealized losses on derivatives accounted for as cash flow hedges										0.0	0.0
Other comprehensive income (loss) of bank subsidiary									90.0	(37.8)	13.4
Total other comprehensive income (loss), net of tax									89.9	(37.8)	13.3
Total comprehensive income									$ 610.3	$ 430.3	$ 350.5