Parent Company Financial Information - Condensed Statements of 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
Revenues:
Securities									$ 186.5	$ 184.2	$ 153.7
Total interest and dividend income	$ 496.9	$ 470.9	$ 472.8	$ 440.5	$ 431.8	$ 390.0	$ 375.4	$ 359.1	1,881.1	1,556.3	1,302.4
Net security losses									0.2	(9.8)	(25.4)
Other non-interest income									85.9	50.9	62.6
Expenses:
Interest on notes and debentures									34.9	33.3	29.9
Non-interest expense	325.7	281.4	278.4	277.2	262.7	241.3	248.6	243.5	1,162.7	996.1	960.3
Income tax expense (benefit)	36.4	30.4	35.2	30.0	15.8	32.2	30.8	29.4	132.0	108.2	129.9
Net income	$ 137.5	$ 135.1	$ 133.2	$ 114.6	$ 132.9	$ 117.0	$ 110.2	$ 107.9	520.4	468.1	337.2
People's United Financial, Inc. [Member]
Revenues:
Securities									0.4	0.4	1.1
Total interest and dividend income									0.4	0.4	1.1
Dividend income from bank subsidiary									457.0	342.0	292.0
Net security losses									0.0	0.0	(1.2)
Other non-interest income									1.7	2.3	16.9
Total revenues									459.1	344.7	308.8
Expenses:
Interest on notes and debentures									19.2	18.7	19.0
Non-interest expense									14.1	11.4	13.9
Total expenses									33.3	30.1	32.9
Income (loss) before income tax expense (benefit)									425.8	314.6	275.9
Income tax expense (benefit)									(6.2)	(5.5)	(5.7)
Income before subsidiaries undistributed income									432.0	320.1	281.6
Subsidiaries undistributed income									88.4	148.0	55.6
Net income									$ 520.4	$ 468.1	$ 337.2