Notes and Debentures - Schedule of Subordinated Borrowing (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Subordinated Borrowing [Line Items]
Total notes and debentures	$ 993.1	$ 895.8
People's United Financial, Inc. [Member]
Subordinated Borrowing [Line Items]
Total notes and debentures	579.9	497.7
People's United Financial, Inc. [Member] \| 3.65% Senior Notes Due 2022 [Member]
Subordinated Borrowing [Line Items]
Senior notes	498.3	497.7
People's United Financial, Inc. [Member] \| 5.75% Subordinated Notes Due 2024 [Member]
Subordinated Borrowing [Line Items]
Subordinated notes	81.6	0.0
People's United Bank [Member] \| 4.00% Subordinated Notes Due 2024 [Member]
Subordinated Borrowing [Line Items]
Subordinated notes	$ 413.2	$ 398.1

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.

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