Parent Company Financial Information - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Flows from Operating Activities:
Net income	$ 520.4	$ 468.1	$ 337.2
Adjustments to reconcile net income to net cash provided by operating activities:
Net security (gains) losses	(0.2)	9.8	25.4
Net change in other assets and other liabilities	(466.5)	(40.0)	38.1
Net cash provided by operating activities	240.8	589.3	603.0
Cash Flows from Investing Activities:
Proceeds from sales of equity securities	1.6	2.3	0.0
Proceeds from sales of debt securities available-for-sale	365.3	313.8	1,305.5
Net cash used in investing activities	(310.5)	(0.4)	(693.5)
Cash Flows from Financing Activities:
Repayment of notes and debentures	0.0	0.0	(125.0)
Cash dividends paid on common stock	(274.8)	(243.8)	(227.9)
Cash dividends paid on preferred stock	(14.1)	(14.1)	(14.1)
Common stock repurchases	(4.5)	(2.5)	(3.4)
Proceeds from stock options exercised	24.1	27.9	61.8
Net cash (used in) provided by financing activities	(61.3)	(539.5)	359.0
Net (decrease) increase in cash, cash equivalents and restricted cash	(131.0)	49.4	268.5
Cash, cash equivalents and restricted cash at beginning of year	932.0	882.6	614.1
Cash, cash equivalents and restricted cash at end of year	801.0	932.0	882.6
People's United Financial, Inc. [Member]
Cash Flows from Operating Activities:
Net income	520.4	468.1	337.2
Adjustments to reconcile net income to net cash provided by operating activities:
Subsidiaries undistributed income	(88.4)	(148.0)	(55.6)
Net security (gains) losses	0.0	0.0	1.2
Net change in other assets and other liabilities	(0.7)	23.1	20.1
Net cash provided by operating activities	431.3	343.2	302.9
Cash Flows from Investing Activities:
Proceeds from sales of equity securities	1.6	2.3	0.0
Proceeds from sales of debt securities available-for-sale	0.0	0.0	75.6
Increase in investment in bank subsidiary	0.0	(200.0)	0.0
Net cash used in investing activities	1.6	(197.7)	75.6
Cash Flows from Financing Activities:
Repayment of notes and debentures	0.0	0.0	(125.0)
Cash dividends paid on common stock	(274.8)	(243.8)	(227.9)
Cash dividends paid on preferred stock	(14.1)	(14.1)	(14.1)
Common stock repurchases	(4.5)	(2.5)	(3.4)
Proceeds from stock options exercised	24.1	27.9	61.8
Net cash (used in) provided by financing activities	(269.3)	(232.5)	(308.6)
Net (decrease) increase in cash, cash equivalents and restricted cash	163.6	(87.0)	69.9
Cash, cash equivalents and restricted cash at beginning of year	291.6	378.6	308.7
Cash, cash equivalents and restricted cash at end of year	$ 455.2	$ 291.6	$ 378.6