Parent Company Financial Information - Condensed Statements of Condition (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets:
Cash at bank subsidiary	$ 484.2	$ 665.7
Total cash and cash equivalents	801.0	932.0
Equity securities, at fair value	8.2	8.1
Goodwill	3,065.5	2,685.7	$ 2,411.4
Other assets	1,853.0	1,091.6
Total assets	58,589.8	47,877.3
Liabilities and Stockholders’ Equity:
Notes and debentures	993.1	895.8
Other liabilities	905.5	695.2
Total stockholders’ equity	7,947.2	6,533.9	$ 5,819.9	$ 5,141.9
Total liabilities and stockholders’ equity	58,589.8	47,877.3
People's United Financial, Inc. [Member]
Assets:
Cash at bank subsidiary	455.2	291.6
Total cash and cash equivalents	455.2	291.6
Equity securities, at fair value	8.2	8.1
Goodwill	203.0	197.1
Due from bank subsidiary	4.6	5.5
Other assets	56.0	45.9
Total assets	8,531.8	7,036.3
Liabilities and Stockholders’ Equity:
Notes and debentures	579.9	497.7
Other liabilities	4.7	4.7
Total stockholders’ equity	7,947.2	6,533.9
Total liabilities and stockholders’ equity	8,531.8	7,036.3
People's United Financial, Inc. [Member] | Bank Subsidiary [Member]
Assets:
Investments in subsidiaries:	7,800.0	6,485.3
People's United Financial, Inc. [Member] | Non Bank Subsidiaries [Member]
Assets:
Investments in subsidiaries:	$ 4.8	$ 2.8