People's United Financial (PBCT) 10-Q 9 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 31, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Trading Symbol	PBCT
Entity Registrant Name	People's United Financial, Inc.
Entity Central Index Key	1,378,946
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		377,364,168
Entity Emerging Growth Company	false
Entity Small Business	false

Consolidated Statements of Cond

Consolidated Statements of Condition - (Unaudited) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets
Cash and due from banks	$ 410.5	$ 505.1
Short-term investments	127.5	377.5
Total cash and cash equivalents (note 2)	538	882.6
Securities (notes 2 and 13):
Trading debt securities, at fair value	8.3	8.2
Equity securities, at fair value	8.9	8.7
Debt securities available-for-sale, at fair value	3,312.1	3,125.3
Debt securities held-to-maturity, at amortized cost (fair value of $3.67 billion and $3.63 billion)	3,742.9	3,588.1
Federal Home Loan Bank and Federal Reserve Bank stock, at cost	312.4	312.3
Total securities	7,384.6	7,042.6
Loans held-for-sale	15.2	16.6
Loans (note 3):
Total loans	32,199.3	32,575.3
Less allowance for loan losses	(238)	(234.4)
Total loans, net	31,961.3	32,340.9
Goodwill (note 6)	2,435.2	2,411.4
Bank-owned life insurance	407.7	405
Premises and equipment, net	243.8	253
Other acquisition-related intangible assets (note 6)	133.7	148.6
Other assets (notes 1, 3 and 11)	1,013.7	952.7
Total assets	44,133.2	44,453.4
Deposits:
Non-interest-bearing	8,060.2	8,002.4
Savings	4,048.8	4,410.5
Interest-bearing checking and money market	15,065.3	15,189.1
Time	6,035.9	5,454.3
Total deposits	33,210.2	33,056.3
Borrowings:
Federal Home Loan Bank advances	2,369.7	2,774.4
Federal funds purchased	735	820
Customer repurchase agreements	261.3	301.6
Other borrowings	26	207.8
Total borrowings	3,392	4,103.8
Notes and debentures	885.6	901.6
Other liabilities (notes 1 and 11)	686.5	571.8
Total liabilities	38,174.3	38,633.5
Commitments and contingencies (notes 1 and 8)
Stockholders' Equity (notes 4, 7 and 13)
Preferred stock ($0.01 par value; 50.0 million shares authorized; 10.0 million shares issued and outstanding at both dates)	244.1	244.1
Common stock ($0.01 par value; 1.95 billion shares authorized; 437.7 million shares and 435.6 million shares issued)	4.4	4.4
Additional paid-in capital	6,054.3	6,012.3
Retained earnings	1,220.9	1,040.2
Unallocated common stock of Employee Stock Ownership Plan, at cost (6.4 million shares and 6.6 million shares)	(131.9)	(137.3)
Accumulated other comprehensive loss	(270.8)	(181.7)
Treasury stock, at cost (89.0 million shares at both dates)	(1,162.1)	(1,162.1)
Total stockholders' equity	5,958.9	5,819.9
Total liabilities and stockholders' equity	44,133.2	44,453.4
Commercial [Member]
Loans (note 3):
Total loans	23,373.4	23,705.2
Less allowance for loan losses	(207.1)	(204.5)
Retail Loans [Member]
Loans (note 3):
Total loans	8,825.9	8,870.1
Less allowance for loan losses	(30.9)	(29.9)
Commercial Real Estate Loan [Member] | Commercial [Member]
Loans (note 3):
Total loans	10,595.5	11,068.7
Commercial and Industrial [Member] | Commercial [Member]
Loans (note 3):
Total loans	8,568.6	8,731.1
Equipment Financing [Member] | Commercial [Member]
Loans (note 3):
Total loans	4,209.3	3,905.4
Residential Mortgage Loan [Member] | Retail Loans [Member]
Loans (note 3):
Total loans	6,911.9	6,805.7
Home Equity and Other Consumer [Member] | Retail Loans [Member]
Loans (note 3):
Total loans	$ 1,914	$ 2,064.4

Consolidated Statements of Co_2

Consolidated Statements of Condition - (Unaudited) (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Securities held to maturity, at fair value	$ 3,666.8	$ 3,633.7
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	50,000,000	50,000,000
Preferred stock, shares issued	10,000,000	10,000,000
Preferred stock, shares outstanding	10,000,000	10,000,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	1,950,000,000	1,950,000,000
Common stock, shares issued	437,700,000	435,600,000
Unallocated common stock of Employee Stock Ownership Plan, shares	6,359,259	6,600,000
Treasury stock, shares	89,000,000	89,000,000

Consolidated Statements of Inco

Consolidated Statements of Income - (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Interest and dividend income:
Commercial real estate	$ 114.7	$ 105.6	$ 333.2	$ 299.5
Residential mortgage	56	52.5	166	154.1
Home equity and other consumer	22	21	64.2	59.3
Total interest on loans	342.1	300.6	984.6	836.2
Securities	46.6	37.2	135.7	112.1
Short-term investments	1.1	1.1	3.6	2.7
Loans held-for-sale	0.2	0.3	0.6	0.7
Total interest and dividend income	390	339.2	1,124.5	951.7
Interest expense:
Deposits	56.9	34.4	145.5	92.4
Borrowings	18.2	12.7	50.9	28.9
Notes and debentures	8.5	7.5	24.7	22.3
Total interest expense	83.6	54.6	221.1	143.6
Net interest income	306.4	284.6	903.4	808.1
Provision for loan losses (note 3)	8.2	7	20.1	18.5
Net interest income after provision for loan losses	298.2	277.6	883.3	789.6
Non-interest income:
Operating lease income	11	10.9	32.9	32.1
Insurance revenue	9.8	9.7	27.9	26.3
Commercial banking lending fees	7.9	7	27.7	26.7
Cash management fees	7	6.8	20.6	19.6
Brokerage commissions	3.2	2.8	9.5	9.2
Net security gains (losses) (note 2)	0.1		0.2	(15.6)
Other non-interest income (note 2)	11	9.9	33.5	44.3
Total non-interest income	92.3	89.3	277.6	265.6
Non-interest expense:
Compensation and benefits (note 13)	135.7	129.9	411.4	389.9
Occupancy and equipment	41.6	40.2	123.6	118.6
Professional and outside services	17	19.2	56.2	62.8
Regulatory assessments	10	10.3	30.5	29.8
Operating lease expense	8.9	8.8	26.6	26.3
Amortization of other acquisition-related intangible assets (note 6)	4.9	7.9	14.9	22.1
Other non-interest expense (note 13)	23.2	20.8	70.2	71
Total non-interest expense	241.3	237.1	733.4	720.5
Income before income tax expense	149.2	129.8	427.5	334.7
Income tax expense (note 1)	32.2	39	92.4	103.8
Net income	117	90.8	335.1	230.9
Preferred stock dividend	3.5	3.5	10.5	10.5
Net income available to common shareholders	$ 113.5	$ 87.3	$ 324.6	$ 220.4
Earnings per common share (note 5):
Basic	$ 0.33	$ 0.26	$ 0.95	$ 0.67
Diluted	$ 0.33	$ 0.26	$ 0.94	$ 0.67
Deposit Account [Member]
Non-interest income:
Bank service charges	$ 24.9	$ 25.3	$ 73	$ 73.8
Investment Advisory, Management and Administrative Service [Member]
Non-interest income:
Bank service charges	17.4	16.9	52.3	49.2
Commercial and Industrial [Member]
Interest and dividend income:
Interest and fee income	93.2	80	265.6	218.7
Equipment Financing [Member]
Interest and dividend income:
Interest and fee income	$ 56.2	$ 41.5	$ 155.6	$ 104.6

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 117	$ 90.8	$ 335.1	$ 230.9
Other comprehensive (loss) income, net of tax:
Net actuarial loss and prior service credit related to pension and other postretirement plans	1.5	0.9	4.5	2.8
Net unrealized gains and losses on debt securities available-for-sale	(12)	3.6	(56.3)	21
Amortization of unrealized losses on debt securities transferred to held-to-maturity	0.7	0.6	2.3	1.6
Net unrealized gains and losses on derivatives accounted for as cash flow hedges	(0.3)	(0.2)	(2.3)	(0.1)
Current period other comprehensive income	(10.1)	4.9	(51.8)	25.3
Total comprehensive income	$ 106.9	$ 95.7	$ 283.3	$ 256.2

Consolidated Statements of Chan

Consolidated Statements of Changes in Stockholders' Equity - (Unaudited) - USD ($) $ in Millions	Total	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Unallocated ESOP Common Stock [Member]	Accumulated Other Comprehensive Loss [Member]	Treasury Stock [Member]
Beginning Balance at Dec. 31, 2016	$ 5,141.9	$ 244.1	$ 4	$ 5,446.1	$ 949.3	$ (144.6)	$ (195)	$ (1,162)
Net income	230.9				230.9
Total other comprehensive income, net of tax (note 4)	25.3						25.3
Common stock issued in Suffolk Bancorp acquisition	484.8		0.2	484.6
Cash dividends on common stock	(169.3)				(169.3)
Cash dividends on preferred stock	(10.5)				(10.5)
Restricted stock and performance-based share awards	9			9.1				(0.1)
Employee Stock Ownership Plan common committed to be released	4.8				(0.7)	5.5
Common stock repurchased and retired upon vesting of restricted stock awards	(3.3)				(3.3)
Stock options exercised	32.5		0.1	32.4
Ending Balance at Sep. 30, 2017	5,746.1	244.1	4.3	5,972.2	996.4	(139.1)	(169.7)	(1,162.1)
Beginning Balance at Dec. 31, 2017	5,819.9	244.1	4.4	6,012.3	1,040.2	(137.3)	(181.7)	(1,162.1)
Net income	335.1				335.1
Total other comprehensive income, net of tax (note 4)	(51.8)						(51.8)
Cash dividends on common stock	(178.8)				(178.8)
Cash dividends on preferred stock	(10.5)				(10.5)
Restricted stock and performance-based share awards	10.8			10.8
Employee Stock Ownership Plan common committed to be released	4.8				(0.6)	5.4
Common stock repurchased and retired upon vesting of restricted stock awards	(2.4)				(2.4)
Stock options exercised	31.2			31.2
Ending Balance at Sep. 30, 2018	5,958.9	$ 244.1	$ 4.4	$ 6,054.3	1,220.9	$ (131.9)	(270.8)	$ (1,162.1)
Transition adjustments related to adoption of new accounting standards (notes 4 and 13)	$ 0.6				$ 37.9		$ (37.3)

Consolidated Statements of Ch_2

Consolidated Statements of Changes in Stockholders' Equity - (Unaudited) (Parenthetical) - $ / shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash dividends on common stock, per share	$ 0.5225	$ 0.5150
Retained Earnings [Member]
Cash dividends on common stock, per share	$ 0.5225	$ 0.5150

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - (Unaudited) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash Flows from Operating Activities:
Net income	$ 335.1	$ 230.9
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization of premises and equipment	26.8	29.3
Expense related to operating leases	26.6	26.3
Provision for loan losses	20.1	18.5
Amortization of other acquisition-related intangible assets	14.9	22.1
Expense related to share-based awards	14.8	13.4
Employee Stock Ownership Plan common stock committed to be released	4.8	4.8
Net security (gains) losses	(0.2)	15.6
Net gains on sales of residential mortgage loans	(0.9)	(2.7)
Originations of loans held-for-sale	(126.8)	(212.5)
Proceeds from sales of loans held-for-sale	129.1	239.5
Net increase in trading debt securities		(1.5)
Excess income tax benefits from stock option exercises	1.4	1.3
Net changes in other assets and other liabilities	14.1	15.6
Net cash provided by operating activities	459.8	400.6
Cash Flows from Investing Activities:
Proceeds from sales of equity securities	1.9
Proceeds from principal repayments and maturities of debt securities available-for-sale	346.9	456.1
Proceeds from sales of debt securities available-for-sale	0.1	1,016.2
Proceeds from principal repayments and maturities of debt securities held-to-maturity	142.1	92.1
Purchases of debt securities available-for-sale	(625.1)	(237.6)
Purchases of debt securities held-to-maturity	(309.6)	(1,235.9)
Net redemptions (purchases) of Federal Reserve Bank stock	7.3	(19.9)
Net (purchases) redemptions of Federal Home Loan Bank stock	(7.4)	17.6
Proceeds from sales of loans	11.9	8.4
Loan disbursements, net of principal collections	423.3	(325.4)
Purchases of premises and equipment	(17.3)	(6.7)
Purchases of leased equipment	(31.3)	(20.5)
Proceeds from sales of real estate owned	8.1	8.6
Return of premium on bank-owned life insurance, net	0.8	1.7
Net cash (paid) acquired in acquisitions	(35.8)	28.9
Net cash used in investing activities	(84.1)	(216.4)
Cash Flows from Financing Activities:
Net increase in deposits	153.9	834.1
Net decrease in borrowings with terms of three months or less	(252.8)	(279.4)
Repayments of borrowings with terms of more than three months	(456.6)	(356.1)
Repayment of notes and debentures		(125)
Cash dividends paid on common stock	(178.8)	(169.3)
Cash dividends paid on preferred stock	(10.5)	(10.5)
Repurchases of common stock	(2.4)	(3.3)
Proceeds from stock options exercised	27.2	28.1
Contingent consideration payments	(0.3)	(0.3)
Net cash used in financing activities	(720.3)	(81.7)
Net (decrease) increase in cash, cash equivalents and restricted cash	(344.6)	102.5
Cash, cash equivalents and restricted cash at beginning of period	882.6	614.1
Cash, cash equivalents and restricted cash at end of period	538	716.6
Supplemental Information:
Interest payments	214.6	138.4
Income tax payments	41.2	100.1
Real estate properties acquired by foreclosure	5.3	9.3
Unsettled purchases of securities	3.8	110.5
Assets acquired and liabilities assumed in acquisitions
Non-cash assets, excluding goodwill and other acquisition-related intangibles	69.1	2,642.1
Liabilities	$ 1.4	2,634.1
Common stock issued in Suffolk Bancorp acquisition		$ 484.8

General

General	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
General	NOTE 1. GENERAL In the opinion of management, the accompanying unaudited consolidated financial statements of People’s United Financial, Inc. (“People’s United” or the “Company”) have been prepared to reflect all adjustments necessary to present fairly the financial position and results of operations as of the dates and for the periods shown. All significant intercompany transactions and balances are eliminated in consolidation. Certain reclassifications have been made to prior period amounts to conform to the current period presentation. In preparing the consolidated financial statements, management is required to make significant estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses. Actual results could differ from management’s current estimates, as a result of changing conditions and future events. Several accounting estimates are particularly critical and are susceptible to significant near-term change, including the allowance for loan losses and asset impairment judgments, such as the recoverability of goodwill and other intangible assets. These accounting estimates are reviewed with the Audit Committee of the Board of Directors. The judgments used by management in applying critical accounting policies may be affected by economic conditions, which may result in changes to future financial results. For example, subsequent evaluations of the loan portfolio, in light of the factors then prevailing, may result in significant changes in the allowance for loan losses in future periods, and the inability to collect outstanding principal may result in increased loan losses. Note 1 to People’s United’s audited consolidated financial statements included in the Annual Report on Form 10-K People’s United holds ownership interests in limited partnerships formed to develop and operate affordable housing units for lower income tenants throughout its franchise area. The underlying partnerships, which are considered variable interest entities, are not consolidated into the Company’s Consolidated Financial Statements. These investments have historically played a role in enabling People’s United Bank, National Association (the “Bank”) to meet its Community Reinvestment Act requirements while, at the same time, providing federal income tax credits. Affordable housing investments, including all legally binding commitments to fund future investments, are included in other assets in the Consolidated Statements of Condition ($286.6 million and $250.7 million at September 30, 2018 and December 31, 2017, respectively). Included in other liabilities in the Consolidated Statements of Condition is a liability for all legally binding unfunded commitments to fund future investments ($112.5 million and $99.6 million at those dates). The cost of the Company’s investments is amortized on a straight-line basis over the period during which the related federal income tax credits are realized (generally ten years). Amortization expense, which is included as a component of income tax expense, totaled $4.8 million and $4.2 million for the three months ended September 30, 2018 and 2017, respectively, and $14.0 million and $12.3 million for the nine months ended September 30, 2018 and 2017, respectively. Certain information and footnote disclosures normally included in consolidated financial statements prepared in conformity with U.S. generally accepted accounting principles (“GAAP”) have been omitted or condensed. As a result, the accompanying consolidated financial statements should be read in conjunction with People’s United’s Annual Report on Form 10-K On June 27, 2018, the Bank completed its all-cash

Cash and Cash Equivalents and S

Cash and Cash Equivalents and Securities	9 Months Ended
	Sep. 30, 2018
Cash and Cash Equivalents [Abstract]
Cash and Cash Equivalents and Securities	NOTE 2. CASH AND CASH EQUIVALENTS AND SECURITIES At December 31, 2017, cash and due from banks included restricted cash totaling $13.8 million (none at September 30, 2018) relating to one remaining securitization (which was repaid in June 2018) assumed in the acquisition of LEAF Commercial Capital, Inc. (“LEAF”). Included in short-term investments are interest-bearing deposits at the Federal Reserve Bank of New York (the “FRB-NY”) The amortized cost, gross unrealized gains and losses, and fair value of People’s United’s debt securities available-for-sale held-to-maturity As of September 30, 2018 (in millions) Amortized Gross Gross Fair Debt securities available-for-sale: U.S. Treasury and agency $ 683.5 $ — $ (31.4 ) $ 652.1 GSE (1) mortgage-backed securities 2,742.5 1.4 (83.9 ) 2,660.0 Total debt securities available-for-sale $ 3,426.0 $ 1.4 $ (115.3 ) $ 3,312.1 Debt securities held-to-maturity: State and municipal $ 2,268.2 $ 19.7 $ (37.4 ) $ 2,250.5 GSE mortgage-backed securities 1,407.3 — (57.7 ) 1,349.6 Corporate 65.9 — (0.7 ) 65.2 Other 1.5 — — 1.5 Total debt securities held-to-maturity $ 3,742.9 $ 19.7 $ (95.8 ) $ 3,666.8 (1) Government sponsored enterprise As of December 31, 2017 (in millions) Amortized Gross Gross Fair Debt securities available-for-sale: U.S. Treasury and agency $ 687.1 $ — $ (18.3 ) $ 668.8 GSE mortgage-backed securities 2,477.8 7.4 (28.7 ) 2,456.5 Total debt securities available-for-sale $ 3,164.9 $ 7.4 $ (47.0 ) $ 3,125.3 Debt securities held-to-maturity: State and municipal $ 2,060.4 $ 64.5 $ (4.6 ) $ 2,120.3 GSE mortgage-backed securities 1,474.9 0.3 (15.4 ) 1,459.8 Corporate 51.3 0.8 — 52.1 Other 1.5 — — 1.5 Total debt securities held-to-maturity $ 3,588.1 $ 65.6 $ (20.0 ) $ 3,633.7 At September 30, 2018 and December 31, 2017, debt securities available-for-sale held-to-maturity The following table is a summary of the amortized cost and fair value of debt securities as of September 30, 2018, based on remaining period to contractual maturity. Information for GSE mortgage-backed securities is based on the final contractual maturity dates without considering repayments and prepayments. Available-for-Sale Held-to-Maturity (in millions) Amortized Fair Amortized Fair U.S. Treasury and agency: Within 1 year $ 6.0 $ 6.0 $ — $ — After 1 but within 5 years 677.5 646.1 — — Total 683.5 652.1 — — GSE mortgage-backed securities: Within 1 year — — 4.3 4.3 After 1 but within 5 years 53.6 52.9 234.7 226.4 After 5 but within 10 years 1,014.6 1,000.8 800.4 764.9 After 10 years 1,674.3 1,606.3 367.9 354.0 Total 2,742.5 2,660.0 1,407.3 1,349.6 State and municipal: Within 1 year — — 23.5 23.5 After 1 but within 5 years — — 180.3 183.2 After 5 but within 10 years — — 334.4 340.1 After 10 years — — 1,730.0 1,703.7 Total — — 2,268.2 2,250.5 Corporate: After 1 but within 5 years — — 5.0 5.0 After 5 but within 10 years — — 60.9 60.2 Total — — 65.9 65.2 Other: After 1 but within 5 years — — 1.5 1.5 Total — — 1.5 1.5 Total: Within 1 year 6.0 6.0 27.8 27.8 After 1 but within 5 years 731.1 699.0 421.5 416.1 After 5 but within 10 years 1,014.6 1,000.8 1,195.7 1,165.2 After 10 years 1,674.3 1,606.3 2,097.9 2,057.7 Total $ 3,426.0 $ 3,312.1 $ 3,742.9 $ 3,666.8 Management conducts a periodic review and evaluation of the securities portfolio to determine if the decline in fair value of any security is deemed to be other-than-temporary. Other-than-temporary impairment losses are recognized on debt securities when: (i) People’s United has an intention to sell the security; (ii) it is more likely than not that People’s United will be required to sell the security prior to recovery; or (iii) People’s United does not expect to recover the entire amortized cost basis of the security. Other-than-temporary impairment losses on debt securities are reflected in earnings as realized losses to the extent the impairment is related to credit losses of the issuer. The amount of the impairment related to other factors is recognized in other comprehensive income. Management has the ability and intent to hold the securities classified as held-to-maturity The following tables summarize those debt securities with unrealized losses, segregated by the length of time the securities have been in a continuous unrealized loss position, at the respective dates. Certain unrealized losses totaled less than $0.1 million. Continuous Unrealized Loss Position Less Than 12 Months 12 Months Or Longer Total As of September 30, 2018 (in millions) Fair Unrealized Fair Unrealized Fair Unrealized Debt securities available-for-sale: GSE mortgage-backed securities $ 893.6 $ (14.2 ) $ 1,604.7 $ (69.7 ) $ 2,498.3 $ (83.9 ) U.S. Treasury and agency 152.5 (1.8 ) 494.6 (29.6 ) 647.1 (31.4 ) Debt securities held-to-maturity: GSE mortgage-backed securities 330.7 (11.9 ) 1,018.9 (45.8 ) 1,349.6 (57.7 ) State and municipal 1,048.1 (21.6 ) 265.4 (15.8 ) 1,313.5 (37.4 ) Corporate 51.4 (0.7 ) — — 51.4 (0.7 ) Total $ 2,476.3 $ (50.2 ) $ 3,383.6 $ (160.9 ) $ 5,859.9 $ (211.1 ) Continuous Unrealized Loss Position Less Than 12 Months 12 Months Or Longer Total As of December 31, 2017 (in millions) Fair Unrealized Fair Unrealized Fair Unrealized Debt securities available-for-sale: GSE mortgage-backed securities $ 1,013.5 $ (8.7 ) $ 1,114.8 $ (20.0 ) $ 2,128.3 $ (28.7 ) U.S. Treasury and agency 156.0 — 507.7 (18.3 ) 663.7 (18.3 ) Debt securities held-to-maturity: GSE mortgage-backed securities 1,289.3 (14.7 ) 45.0 (0.7 ) 1,334.3 (15.4 ) State and municipal 106.2 (0.5 ) 224.9 (4.1 ) 331.1 (4.6 ) Total $ 2,565.0 $ (23.9 ) $ 1,892.4 $ (43.1 ) $ 4,457.4 $ (67.0 ) At September 30, 2018, approximately 56% of the 2,137 debt securities owned by the Company, consisting of 151 debt securities classified as available-for-sale held-to-maturity, The cause of the gross unrealized losses with respect to all of the debt securities is directly related to changes in interest rates. At this time, management does not intend to sell such securities nor is it more likely than not, based upon available evidence, that management will be required to sell such securities prior to recovery. As such, management believes that all gross unrealized losses within the securities portfolio at September 30, 2018 represent temporary impairments. No other-than-temporary impairment losses were recognized in the Consolidated Statements of Income for the three or nine months ended September 30, 2018 or 2017. Security transactions are recorded on the trade date. Realized gains and losses are determined using the specific identification method and reported in non-interest Effective January 1, 2018, People’s United adopted new accounting guidance that requires equity investments (other than equity method investments) be measured at fair value with changes in fair value recognized in net income (see Note 13). At December 31, 2017, the Company’s securities portfolio included equity securities with an amortized cost of $9.6 million and a fair value of $8.7 million. Accordingly, upon adoption of this guidance, a cumulative-effect transition adjustment, representing the cumulative unrealized loss (net-of-tax) non-interest The Bank, as a member of the Federal Home Loan Bank (the “FHLB”) of Boston, is currently required to purchase and hold shares of capital stock in the FHLB of Boston (total cost of $134.1 million and $130.0 million at September 30, 2018 and December 31, 2017, respectively) in an amount equal to its membership base investment plus an activity based investment determined according to the Bank’s level of outstanding FHLB advances. As a result of prior acquisitions, the Bank acquired shares of capital stock in the FHLB of New York (total cost of $0.7 million and $12.0 million at September 30, 2018 and December 31, 2017, respectively). Based on the current capital adequacy and liquidity position of both the FHLB of Boston and the FHLB of New York, management believes there is no impairment in the Company’s investment at September 30, 2018 and the cost of the investment approximates fair value. The Bank, as a member of the Federal Reserve Bank system, is currently required to purchase and hold shares of capital stock in the FRB-NY FRB-NY,

Loans

Loans	9 Months Ended
	Sep. 30, 2018
Receivables [Abstract]
Loans	NOTE 3. LOANS For purposes of disclosures related to the credit quality of financing receivables and the allowance for loan losses, People’s United has identified two loan portfolio segments, Commercial and Retail, which are comprised of the following loan classes: • Commercial Portfolio • Retail Portfolio Loans acquired in connection with business combinations are referred to as ‘acquired’ loans as a result of the manner in which they are accounted for (see further discussion under ‘Acquired Loans’). All other loans are referred to as ‘originated’ loans. Accordingly, selected credit quality disclosures that follow are presented separately for the ‘originated’ loan portfolio and the ‘acquired’ loan portfolio. People’s United maintains several significant accounting policies with respect to loans, including: • Establishment of the allowance for loan losses (including the identification of ‘impaired’ loans and related impairment measurement considerations); • Income recognition (including the classification of a loan as ‘non-accrual’ • Recognition of loan charge-offs. The Company did not change its application of the accounting policies noted above or its methodology for determining the allowance for loan losses during the nine months ended September 30, 2018. The following table summarizes People’s United’s loans by loan portfolio segment and class: September 30, 2018 December 31, 2017 (in millions) Originated Acquired Total Originated Acquired Total Commercial: Commercial real estate $ 9,750.9 $ 844.6 $ 10,595.5 $ 10,126.6 $ 942.1 $ 11,068.7 Commercial and industrial 8,078.9 489.7 8,568.6 8,129.9 601.2 8,731.1 Equipment financing 3,734.9 474.4 4,209.3 3,308.5 596.9 3,905.4 Total Commercial Portfolio 21,564.7 1,808.7 23,373.4 21,565.0 2,140.2 23,705.2 Retail: Residential mortgage: Adjustable-rate 5,810.8 126.7 5,937.5 5,782.6 144.0 5,926.6 Fixed-rate 867.5 106.9 974.4 758.0 121.1 879.1 Total residential mortgage 6,678.3 233.6 6,911.9 6,540.6 265.1 6,805.7 Home equity and other consumer: Home equity 1,823.6 43.6 1,867.2 1,960.0 55.2 2,015.2 Other consumer 44.2 2.6 46.8 45.6 3.6 49.2 Total home equity and other consumer 1,867.8 46.2 1,914.0 2,005.6 58.8 2,064.4 Total Retail Portfolio 8,546.1 279.8 8,825.9 8,546.2 323.9 8,870.1 Total loans $ 30,110.8 $ 2,088.5 $ 32,199.3 $ 30,111.2 $ 2,464.1 $ 32,575.3 Loan origination fees and certain direct loan origination costs are deferred, and the net fee or cost is recognized in interest income as an adjustment of yield. Depending on the loan portfolio, amounts are amortized or accreted using the level yield method over either the actual life or the estimated average life of the loan. Net deferred loan costs, which are included in loans by respective class and accounted for as interest yield adjustments, totaled $91.7 million at September 30, 2018 and $80.4 million at December 31, 2017. The following tables present a summary, by loan portfolio segment, of activity in the allowance for loan losses for the three and nine months ended September 30, 2018 and 2017. With respect to the originated portfolio, an allocation of a portion of the allowance to one segment does not preclude its availability to absorb losses in another segment. Three months ended Commercial Retail September 30, 2018 (in millions) Originated Acquired Total Originated Acquired Total Total Balance at beginning of period $ 202.0 $ 3.8 $ 205.8 $ 30.8 $ 0.2 $ 31.0 $ 236.8 Charge-offs (5.7 ) (2.0 ) (7.7 ) (0.7 ) — (0.7 ) (8.4 ) Recoveries 0.6 0.4 1.0 0.4 — 0.4 1.4 Net loan charge-offs (5.1 ) (1.6 ) (6.7 ) (0.3 ) — (0.3 ) (7.0 ) Provision for loan losses 6.3 1.7 8.0 0.2 — 0.2 8.2 Balance at end of period $ 203.2 $ 3.9 $ 207.1 $ 30.7 $ 0.2 $ 30.9 $ 238.0 Nine months ended Commercial Retail September 30, 2018 (in millions) Originated Acquired Total Originated Acquired Total Total Balance at beginning of period $ 201.1 $ 3.4 $ 204.5 $ 29.7 $ 0.2 $ 29.9 $ 234.4 Charge-offs (12.9 ) (6.3 ) (19.2 ) (2.6 ) — (2.6 ) (21.8 ) Recoveries 2.6 1.0 3.6 1.7 — 1.7 5.3 Net loan charge-offs (10.3 ) (5.3 ) (15.6 ) (0.9 ) — (0.9 ) (16.5 ) Provision for loan losses 12.4 5.8 18.2 1.9 — 1.9 20.1 Balance at end of period $ 203.2 $ 3.9 $ 207.1 $ 30.7 $ 0.2 $ 30.9 $ 238.0 Three months ended Commercial Retail September 30, 2017 (in millions) Originated Acquired Total Originated Acquired Total Total Balance at beginning of period $ 198.3 $ 3.6 $ 201.9 $ 29.6 $ 0.1 $ 29.7 $ 231.6 Charge-offs (4.0 ) (1.0 ) (5.0 ) (1.8 ) — (1.8 ) (6.8 ) Recoveries 0.9 0.1 1.0 0.6 — 0.6 1.6 Net loan charge-offs (3.1 ) (0.9 ) (4.0 ) (1.2 ) — (1.2 ) (5.2 ) Provision for loan losses 4.3 1.3 5.6 1.3 0.1 1.4 7.0 Balance at end of period $ 199.5 $ 4.0 $ 203.5 $ 29.7 $ 0.2 $ 29.9 $ 233.4 Nine months ended Commercial Retail September 30, 2017 (in millions) Originated Acquired Total Originated Acquired Total Total Balance at beginning of period $ 198.8 $ 6.1 $ 204.9 $ 24.2 $ 0.2 $ 24.4 $ 229.3 Charge-offs (12.0 ) (2.9 ) (14.9 ) (5.1 ) — (5.1 ) (20.0 ) Recoveries 3.8 0.1 3.9 1.7 — 1.7 5.6 Net loan charge-offs (8.2 ) (2.8 ) (11.0 ) (3.4 ) — (3.4 ) (14.4 ) Provision for loan losses 8.9 0.7 9.6 8.9 8.9 18.5 Balance at end of period $ 199.5 $ 4.0 $ 203.5 $ 29.7 $ 0.2 $ 29.9 $ 233.4 The following tables summarize, by loan portfolio segment and impairment methodology, the allowance for loan losses and related portfolio balances: As of September 30, 2018 Commercial Retail Total (in millions) Portfolio Allowance Portfolio Allowance Portfolio Allowance Originated loans: Collectively evaluated for impairment $ 21,441.6 $ 195.2 $ 8,458.9 $ 28.4 $ 29,900.5 $ 223.6 Individually evaluated for impairment 123.1 8.0 87.2 2.3 210.3 10.3 Acquired loans: PCI (1) 284.3 2.2 110.3 0.1 394.6 2.3 Purchased performing: Collectively evaluated for impairment 1,521.2 1.7 167.9 — 1,689.1 1.7 Individually evaluated for impairment 3.2 — 1.6 0.1 4.8 0.1 Total $ 23,373.4 $ 207.1 $ 8,825.9 $ 30.9 $ 32,199.3 $ 238.0 As of December 31, 2017 Commercial Retail Total (in millions) Portfolio Allowance Portfolio Allowance Portfolio Allowance Originated loans: Collectively evaluated for impairment $ 21,423.8 $ 196.5 $ 8,454.1 $ 27.3 $ 29,877.9 $ 223.8 Individually evaluated for impairment 141.2 4.6 92.1 2.4 233.3 7.0 Acquired loans: PCI (1) 370.4 2.8 128.1 0.2 498.5 3.0 Purchased performing: Collectively evaluated for impairment 1,769.8 0.6 193.9 — 1,963.7 0.6 Individually evaluated for impairment — — 1.9 — 1.9 — Total $ 23,705.2 $ 204.5 $ 8,870.1 $ 29.9 $ 32,575.3 $ 234.4 (1) Purchased credit impaired (“PCI”) loans are evaluated for impairment on a pool basis. The recorded investments, by class of loan, in originated non-performing September 30, December 31, (in millions) 2018 2017 Commercial: Commercial real estate $ 17.2 $ 23.7 Commercial and industrial 44.9 32.6 Equipment financing 49.3 44.3 Total (1) 111.4 100.6 Retail: Residential mortgage 32.0 32.7 Home equity 14.6 15.4 Other consumer 0.1 — Total (2) 46.7 48.1 Total $ 158.1 $ 148.7 (1) Reported net of government guarantees totaling $2.5 million and $3.1 million at September 30, 2018 and December 31, 2017, respectively. These government guarantees relate, almost entirely, to guarantees provided by the Small Business Administration as well as selected other Federal agencies and represent the carrying value of the loans that are covered by such guarantees, the extent of which (i.e. full or partial) varies by loan. At September 30, 2018, the principal loan classes to which these government guarantees relate are commercial and industrial loans (93%) and commercial real estate loans (7%). (2) Includes $17.6 million and $15.2 million of loans in the process of foreclosure at September 30, 2018 and December 31, 2017, respectively. The preceding table excludes acquired loans that are (i) accounted for as PCI loans and/or (ii) covered by a Federal Deposit Insurance Corporation (“FDIC”) loss-share agreement (“LSA”), which totaled $23.7 million and $25.1 million at September 30, 2018 and December 31, 2017, respectively. Such loans otherwise meet People’s United’s definition of a non-performing non-performing A loan is generally considered “non-performing” non-accrual non-accrual non-accrual All previously accrued but unpaid interest on non-accrual non-accrual non-accrual A loan is considered impaired when, based on current information and events, it is probable that the Company will be unable to collect all amounts due in accordance with the original contractual terms of the loan agreement, including scheduled principal and interest payments. Impaired loans also include certain loans whose terms have been modified in such a way that they are considered troubled debt restructurings (“TDRs”). Loans are considered TDRs if the borrower is experiencing financial difficulty and is afforded a concession by People’s United, such as, but not limited to: (i) payment deferral; (ii) a reduction of the stated interest rate for the remaining contractual life of the loan; (iii) an extension of the loan’s original contractual term at a stated interest rate lower than the current market rate for a new loan with similar risk; (iv) capitalization of interest; or (v) forgiveness of principal or interest. TDRs may either be accruing or placed on non-accrual non-performing non-accrual non-performing Impairment is evaluated on a collective basis for smaller-balance loans with similar credit risk and on an individual loan basis for other loans. If a loan is deemed to be impaired, a specific valuation allowance is allocated, if necessary, so that the loan is reported (net of the allowance) at the present value of expected future cash flows discounted at the loan’s original effective interest rate or at the fair value of the collateral less cost to sell if repayment is expected solely from the collateral. Interest payments on impaired non-accrual At September 30, 2018 and December 31, 2017, People’s United’s recorded investment in loans classified as TDRs totaled $177.4 million and $186.9 million, respectively. The related allowance for loan losses was $5.4 million at September 30, 2018 and $4.4 million at December 31, 2017. Interest income recognized on TDRs totaled $1.2 million and $1.4 million for the three months ended September 30, 2018 and 2017, respectively, and $4.0 million and $3.7 million for the nine months ended September 30, 2018 and 2017, respectively. Fundings under commitments to lend additional amounts to borrowers with loans classified as TDRs were immaterial for the three and nine months ended September 30, 2018 and 2017. Loans that were modified and classified as TDRs during the three and nine months ended September 30, 2018 and 2017 principally involve reduced payment and/or payment deferral, extension of term (generally no more than two years for commercial loans and five years for retail loans) and/or a temporary reduction of interest rate (generally less than 200 basis points). The following tables summarize, by class of loan, the recorded investments in loans modified as TDRs during the three and nine months ended September 30, 2018 and 2017. For purposes of this disclosure, recorded investments represent amounts immediately prior to and subsequent to the restructuring. Three Months Ended September 30, 2018 Pre-Modification Post-Modification Outstanding Outstanding Number Recorded Recorded (dollars in millions) of Contracts Investment Investment Commercial: Commercial real estate (1) 4 $ 20.7 $ 20.7 Commercial and industrial (2) 10 10.3 10.3 Equipment financing (3) 5 10.3 10.3 Total 19 41.3 41.3 Retail: Residential mortgage (4) 4 1.4 1.4 Home equity (5) 19 2.2 2.2 Other consumer — — — Total 23 3.6 3.6 Total 42 $ 44.9 $ 44.9 (1) Represents the following concessions: extension of term (3 contracts; recorded investment of $20.2 million); or reduced payment and/or payment deferral (1 contract; recorded investment of $0.5 million). (2) Represents the following concessions: extension of term (6 contracts; recorded investment of $3.6 million); reduced payment and/or payment deferral (2 contracts; recorded investment of $6.4 million); or a combination of concessions (2 contracts; recorded investment of $0.3 million). (3) Represents the following concessions: extension of term (2 contracts; recorded investment of $3.3 million); reduced payment and/or payment deferral (2 contracts; recorded investment of $5.9 million); or a combination of concessions (1 contract; recorded investment of $1.1 million). (4) Represents the following concessions: loans restructured through bankruptcy (2 contracts; recorded investment of $0.6 million); reduced payment and/or payment deferral (1 contract; recorded investment of $0.7 million); or a combination of concessions (1 contract; recorded investment of $0.1 million). (5) Represents the following concessions: loans restructured through bankruptcy (11 contracts; recorded investment of $1.1 million); reduced payment and/or payment deferral (6 contracts; recorded investment of $1.0 million); or a combination of concessions (2 contracts; recorded investment of $0.1 million). Nine Months Ended September 30, 2018 (dollars in millions) Number Pre-Modification Post-Modification Commercial: Commercial real estate (1) 9 $ 24.3 $ 24.3 Commercial and industrial (2) 34 55.2 55.2 Equipment financing (3) 16 20.4 20.4 Total 59 99.9 99.9 Retail: Residential mortgage (4) 16 4.9 4.9 Home equity (5) 56 4.9 4.9 Other consumer — — — Total 72 9.8 9.8 Total 131 $ 109.7 $ 109.7 (1) Represents the following concessions: extension of term (8 contracts; recorded investment of $23.8 million); or reduced payment and/or payment deferral (1 contract; recorded investment of $0.5 million). (2) Represents the following concessions: extension of term (21 contracts; recorded investment of $30.7 million); reduced payment and/or payment deferral (10 contracts; recorded investment of $23.8 million); or a combination of concessions (3 contracts; recorded investment of $0.7 million). (3) Represents the following concessions: extension of term (2 contracts; recorded investment of $3.3 million); reduced payment and/or payment deferral (8 contracts; recorded investment of $12.9 million); or a combination of concessions (6 contracts; recorded investment of $4.2 million). (4) Represents the following concessions: loans restructured through bankruptcy (5 contracts; recorded investment of $0.9 million); reduced payment and/or payment deferral (6 contracts; recorded investment of $2.4 million); or a combination of concessions (5 contracts; recorded investment of $1.6 million). (5) Represents the following concessions: loans restructured through bankruptcy (37 contracts; recorded investment of $2.8 million); reduced payment and/or payment deferral (10 contracts; recorded investment of $1.3 million); or a combination of concessions (9 contracts; recorded investment of $0.8 million). Three Months Ended September 30, 2017 (dollars in millions) Number Pre-Modification Post-Modification Commercial: Commercial real estate (1) 1 $ 0.1 $ 0.1 Commercial and industrial (2) 9 11.6 11.6 Equipment financing (3) 13 8.1 8.1 Total 23 19.8 19.8 Retail: Residential mortgage (4) 17 3.8 3.8 Home equity (5) 19 1.5 1.5 Other consumer — — — Total 36 5.3 5.3 Total 59 $ 25.1 $ 25.1 (1) Represents the following concession: extension of term (1 contract; recorded investment of $0.1 million). (2) Represents the following concessions: extension of term (9 contracts; recorded investment of $11.6 million). (3) Represents the following concessions: reduced payment and/or payment deferral (12 contracts; recorded investment of $8.0 million); or a combination of concessions (1 contract; recorded investment of $0.1 million). (4) Represents the following concessions: loans restructured through bankruptcy (9 contracts; recorded investment of $1.7 million); reduced payment and/or payment deferral (5 contract; recorded investment of $1.7 million); or a combination of concessions (3 contracts; recorded investment of $0.4 million). (5) Represents the following concessions: loans restructured through bankruptcy (10 contracts; recorded investment of $1.0 million); reduced payment and/or payment deferral (4 contracts; recorded investment of $0.3 million); or a combination of concessions (5 contracts; recorded investment of $0.2 million). Nine Months Ended September 30, 2017 (dollars in millions) Number Pre-Modification Post-Modification Commercial: Commercial real estate (1) 8 $ 5.3 $ 5.3 Commercial and industrial (2) 28 39.6 39.6 Equipment financing (3) 54 25.1 25.1 Total 90 70.0 70.0 Retail: Residential mortgage (4) 45 10.8 10.8 Home equity (5) 65 4.5 4.5 Other consumer — — — Total 110 15.3 15.3 Total 200 $ 85.3 $ 85.3 (1) Represents the following concessions: extension of term (5 contracts; recorded investment of $1.4 million); reduced payment and/or payment deferral (2 contracts; recorded investment of $2.2 million); or a temporary rate reduction (1 contract; recorded investment of $1.7 million). (2) Represents the following concessions: extension of term (25 contracts; recorded investment of $29.9 million); reduced payment and/or payment deferral (2 contracts; recorded investment of $9.4 million); or a combination of concessions (1 contract; recorded investment of $0.3 million). (3) Represents the following concessions: reduced payment and/or payment deferral (28 contracts; recorded investment of $17.4 million); or a combination of concessions (26 contracts; recorded investment of $7.7 million). (4) Represents the following concessions: loans restructured through bankruptcy (28 contracts; recorded investment of $4.2 million); reduced payment and/or payment deferral (10 contracts; recorded investment of $3.4 million); or a combination of concessions (7 contracts; recorded investment of $3.2 million). (5) Represents the following concessions: loans restructured through bankruptcy (41 contracts; recorded investment of $2.6 million); reduced payment and/or payment deferral (10 contracts; recorded investment of $0.8 million); or a combination of concessions (14 contracts; recorded investment of $1.1 million). The following is a summary, by class of loan, of information related to TDRs completed within the previous 12 months that subsequently defaulted during the three and nine months ended September 30, 2018 and 2017. For purposes of this disclosure, the previous 12 months is measured from October 1 of the respective prior year and a default represents a previously-modified loan that became past due 30 days or more during the three or nine months ended September 30, 2018 or 2017. Three Months Ended September 30, 2018 2017 (dollars in millions) Number Recorded Number Recorded Commercial: Commercial real estate — $ — — $ — Commercial and industrial 4 5.0 1 0.1 Equipment financing 1 0.2 8 4.2 Total 5 5.2 9 4.3 Retail: Residential mortgage 5 0.9 2 0.9 Home equity 8 0.6 5 0.8 Other consumer — — — — Total 13 1.5 7 1.7 Total 18 $ 6.7 16 $ 6.0 Nine Months Ended September 30, 2018 2017 (dollars in millions) Number Recorded Number Recorded Commercial: Commercial real estate 2 $ 0.8 — $ — Commercial and industrial 13 8.5 4 1.5 Equipment financing 7 6.8 15 6.8 Total 22 16.1 19 8.3 Retail: Residential mortgage 7 1.3 9 2.7 Home equity 9 0.6 11 1.3 Other consumer — — — — Total 16 1.9 20 4.0 Total 38 $ 18.0 39 $ 12.3 People’s United’s impaired loans consist of certain loans that have been placed on non-accrual As of September 30, 2018 As of December 31, 2017 (in millions) Unpaid Recorded Related Unpaid Recorded Related Without a related allowance for loan losses: Commercial: Commercial real estate $ 31.2 $ 29.2 $ — $ 37.7 $ 36.3 $ — Commercial and industrial 29.3 27.1 — 27.9 25.5 — Equipment financing 31.3 28.0 — 36.9 32.8 — Retail: Residential mortgage 64.0 56.8 — 67.6 60.8 — Home equity 23.5 19.9 — 24.0 20.2 — Other consumer — — — — — — Total $ 179.3 $ 161.0 $ — $ 194.1 $ 175.6 $ — With a related allowance for loan losses: Commercial: Commercial real estate $ 10.3 $ 7.4 $ 0.8 $ 11.7 $ 9.9 $ 0.9 Commercial and industrial 25.9 22.7 4.2 26.9 26.0 2.6 Equipment financing 12.6 11.9 3.0 11.6 10.7 1.1 Retail: Residential mortgage 10.5 10.5 1.8 11.4 11.4 1.7 Home equity 1.6 1.6 0.6 1.7 1.6 0.7 Other consumer — — — — — — Total $ 60.9 $ 54.1 $ 10.4 $ 63.3 $ 59.6 $ 7.0 Total impaired loans: Commercial: Commercial real estate $ 41.5 $ 36.6 $ 0.8 $ 49.4 $ 46.2 $ 0.9 Commercial and industrial 55.2 49.8 4.2 54.8 51.5 2.6 Equipment financing 43.9 39.9 3.0 48.5 43.5 1.1 Total 140.6 126.3 8.0 152.7 141.2 4.6 Retail: Residential mortgage 74.5 67.3 1.8 79.0 72.2 1.7 Home equity 25.1 21.5 0.6 25.7 21.8 0.7 Other consumer — — — — — — Total 99.6 88.8 2.4 104.7 94.0 2.4 Total $ 240.2 $ 215.1 $ 10.4 $ 257.4 $ 235.2 $ 7.0 The following tables summarize, by class of loan, the average recorded investment and interest income recognized on impaired loans for the periods indicated. The average recorded investment amounts are based on month-end Three Months Ended September 30, 2018 2017 (in millions) Average Interest Average Interest Commercial: Commercial real estate $ 36.2 $ 0.4 $ 61.8 $ 0.3 Commercial and industrial 52.4 0.2 59.7 0.7 Equipment financing 36.0 0.1 46.5 0.2 Total 124.6 0.7 168.0 1.2 Retail: Residential mortgage 68.6 0.4 72.4 0.4 Home equity 20.7 0.2 21.8 0.1 Other consumer — — — — Total 89.3 0.6 94.2 0.5 Total $ 213.9 $ 1.3 $ 262.2 $ 1.7 Nine Months Ended September 30, 2018 2017 (in millions) Average Interest Average Interest Commercial: Commercial real estate $ 40.7 $ 0.9 $ 56.9 $ 0.9 Commercial and industrial 50.6 1.3 66.1 1.6 Equipment financing 39.8 0.2 43.0 0.4 Total 131.1 2.4 166.0 2.9 Retail: Residential mortgage 69.5 1.3 71.9 1.3 Home equity 21.0 0.4 21.1 0.3 Other consumer — — — — Total 90.5 1.7 93.0 1.6 Total $ 221.6 $ 4.1 $ 259.0 $ 4.5 The following tables summarize, by class of loan, aging information for originated loans: Past Due As of September 30, 2018 (in millions) Current 30-89 90 Days Total Total Commercial: Commercial real estate $ 9,728.2 $ 10.1 $ 12.6 $ 22.7 $ 9,750.9 Commercial and industrial 8,044.5 14.2 20.2 34.4 8,078.9 Equipment financing 3,642.4 75.6 16.9 92.5 3,734.9 Total 21,415.1 99.9 49.7 149.6 21,564.7 Retail: Residential mortgage 6,625.3 31.9 21.1 53.0 6,678.3 Home equity 1,810.6 6.4 6.6 13.0 1,823.6 Other consumer 44.0 0.1 0.1 0.2 44.2 Total 8,479.9 38.4 27.8 66.2 8,546.1 Total originated loans $ 29,895.0 $ 138.3 $ 77.5 $ 215.8 $ 30,110.8 Included in the “Current” and “30-89 non-performing non-accrual Past Due As of December 31, 2017 (in millions) Current 30-89 90 Days Total Total Commercial: Commercial real estate $ 10,102.3 $ 11.0 $ 13.3 $ 24.3 $ 10,126.6 Commercial and industrial 8,099.0 14.9 16.0 30.9 8,129.9 Equipment financing 3,219.7 83.1 5.7 88.8 3,308.5 Total 21,421.0 109.0 35.0 144.0 21,565.0 Retail: Residential mortgage 6,487.3 32.8 20.5 53.3 6,540.6 Home equity 1,945.2 7.4 7.4 14.8 1,960.0 Other consumer 45.3 0.3 — 0.3 45.6 Total 8,477.8 40.5 27.9 68.4 8,546.2 Total originated loans $ 29,898.8 $ 149.5 $ 62.9 $ 212.4 $ 30,111.2 Included in the “Current” and “30-89 non-performing non-accrual Commercial Credit Quality Indicators The Company utilizes an internal loan risk rating system as a means of monitoring portfolio credit quality and identifying both problem and potential problem loans. Under the Company’s risk rating system, loans not meeting the criteria for problem and potential problem loans as specified below are considered to be “Pass”-rated loans. Problem and potential problem loans are classified as either “Special Mention,” “Substandard” or “Doubtful.” Loans that do not currently expose the Company to sufficient enough risk of loss to warrant classification as either Substandard or Doubtful, but possess weaknesses that deserve management’s close attention, are classified as Special Mention. Substandard loans represent those credits characterized by the distinct possibility that the Company will sustain some loss if the deficiencies are not corrected. Loans classified as Doubtful possess all the weaknesses inherent in those classified Substandard with the added characteristic that collection or liquidation in full, on the basis of existing facts, conditions and values, is highly questionable and/or improbable. Risk ratings on commercial loans are subject to ongoing monitoring by lending and credit personnel with such ratings updated annually or more frequently, if warranted. The Company’s internal Loan Review function is responsible for independently evaluating the appropriateness of those credit risk ratings in connection with its cyclical reviews, the approach to which is risk-based and determined by reference to underlying portfolio credit quality and the results of prior reviews. Differences in risk ratings noted in conjunction with such periodic portfolio loan reviews, if any, are reported to management each month. Retail Credit Quality Indicators Pools of smaller-balance, homogeneous loans with similar risk and loss characteristics are also assessed for probable losses. These loan pools include residential mortgage, home equity and other consumer loans that are not assigned individual loan risk ratings. Rather, the assessment of these portfolios is based upon a consideration of recent historical loss experience, broader portfolio indicators, including trends in delinquencies, non-performing The portfolio-specific risk characteristics considered include: (i) collateral values/loan-to-value non-stated non-owner For example, to the extent LTV ratios exceed 70% (reflecting a weaker collateral position for the Company) or borrower FICO scores are less than 680 (reflecting weaker financial standing and/or credit history of the customer), the loans are considered to have an increased level of inherent loss. As a result, a loan with a combination of these characteristics would generally be classified as “High” risk. Conversely, as LTV ratios decline (reflecting a stronger collateral position for the Company) or borrower FICO scores exceed 680 (reflecting stronger financial standing and/or credit history of the customer), the loans are considered to have a decreased level of inherent loss. A loan with a combination of these characteristics would generally be classified as “Low” risk. This analysis also considers (i) the extent of underwriting that occurred at the time of origination (direct income verification provides further support for credit decisions) and (ii) the property’s intended use (owner-occupied properties are less likely to default compared to ‘investment-type’ non-owner LTV ratios and FICO scores are determined at origination and updated periodically throughout the life of the loan. LTV ratios are updated for loans 90 days past due and FICO scores are updated for the entire portfolio quarterly. The portfolio stratification (“High”, “Moderate” and “Low” risk) and identification of the corresponding credit quality indicators also occurs quarterly. Commercial and Retail loans are also evaluated to determine whether they are impaired loans. Such loans are included in the tabular disclosures of credit quality indicators that follow. Acquired Loan Credit Quality Indicators Upon acquiring a loan portfolio, the Company’s internal Loan Review function undertakes the process of assigning risk ratings to all commercial loans in accordance with the Company’s established policy, which may differ in certain respects from the risk rating policy of the predecessor company. The length of time necessary to complete this process varies based on the size of the acquired portfolio, the quality of the documentation maintained in the underlying loan files and the extent to which the predecessor company followed a risk rating approach comparable to People’s United’s. As a result, while acquired loans are risk rated, there are occasions when such ratings may be deemed “preliminary” until the Company’s re-rating The following is a summary, by class of loan, of credit quality indicators: As of September 30, 2018 (in millions) Commercial Commercial Equipment Total Commercial: Originated loans: Pass $ 9,500.0 $ 7,644.2 $ 3,352.3 $ 20,496.5 Special mention 134.2 170.1 68.5 372.8 Substandard 115.9 264.2 314.1 694.2 Doubtful 0.8 0.4 — 1.2 Total originated loans 9,750.9 8,078.9 3,734.9 21,564.7 Acquired loans: Pass 779.9 425.8 468.1 1,673.8 Special mention 26.2 9.3 3.2 38.7 Substandard 38.5 54.6 3.1 96.2 Doubtful — — — — Total acquired loans 844.6 489.7 474.4 1,808.7 Total $ 10,595.5 $ 8,568.6 $ 4,209.3 $ 23,373.4 As of September 30, 2018 (in millions) Residential Home Other Total Retail: Originated loans: Low risk $ 3,343.1 $ 862.0 $ 28.2 $ 4,233.3 Moderate risk 2,829.0 589.6 6.6 3,425.2 High risk 506.2 372.0 9.4 887.6 Total originated loans 6,678.3 1,823.6 44.2 8,546.1 Acquired loans: Low risk 136.6 — — 136.6 Moderate risk 52.3 — — 52.3 High risk 44.7 43.6 2.6 90.9 Total acquired loans 233.6 43.6 2.6 279.8 Total $ 6,911.9 $ 1,867.2 $ 46.8 $ 8,825.9 As of December 31, 2017 (in millions) Commercial Commercial Equipment Total Commercial: Originated loans: Pass $ 9,859.3 $ 7,760.7 $ 2,899.9 $ 20,519.9 Special mention 159.4 124.0 91.8 375.2 Substandard 107.0 244.2 316.8 668.0 Doubtful 0.9 1.0 — 1.9 Total originated loans 10,126.6 8,129.9 3,308.5 21,565.0 Acquired loans: Pass 892.0 520.0 596.9 2,008.9 Special mention 14.8 15.2 — 30.0 Substandard 35.3 66.0 — 101.3 Doubtful — — — — Total acquired loans 942.1 601.2 596.9 2,140.2 Total $ 11,068.7 $ 8,731.1 $ 3,905.4 $ 23,705.2 As of December 31, 2017 (in millions) Residential Home Other Total Retail: Originated loans: Low risk $ 3,292.1 $ 925.6 $ 28.2 $ 4,245.9 Moderate risk 2,738.8 640.0 7.1 3,385.9 High risk 509.7 394.4 10.3 914.4 Total originated loans 6,540.6 1,960.0 45.6 8,546.2 Acquired loans: Low risk 148.0 — — 148.0 Moderate risk 65.7 — — 65.7 High risk 51.4 55.2 3.6 110.2 Total acquired loans 265.1 55.2 3.6 323.9 Total $ 6,805.7 $ 2,015.2 $ 49.2 $ 8,870.1 Acquired Loans Loans acquired in a business combination are initially recorded at fair value with no carryover of an acquired entity’s previously established allowance for loan losses. Fair value of the loans is determined using market participant assumptions in estimating the amount and timing of both principal and interest cash flows expected to be collected, as adjusted for an estimate of future credit losses and prepayments, and then applying a market-based discount rate to those cash flows. Acquired loans are evaluated upon acquisition and classified as either purchased performing or PCI. For purchased performing loans, any premium or discount, representing the difference between the fair value and the outstanding principal balance of the loans, is recognized (using the level yield method) as an adjustment to interest income over the remaining period to contractual maturity or until the loan is repaid in full or sold. Subsequent to the acquisition date, the method utilized to estimate the required allowance for loan losses for these loans is similar to that for originated loans. However, a provision for loan losses is only recorded when the required allowance for loan losses exceeds any remaining purchase discount at the loan level. PCI loans represent those acquired loans with specific evidence of deterioration in credit quality since origination and for which it is probable that, as of the acquisition date, all contractually required principal and interest payments will not be collected. Such loans are generally accounted for on a pool basis, with pools formed based on the loans’ common risk characteristics, such as loan collateral type and accrual status. Each pool is accounted for as a single asset with a single composite interest rate and an aggregate expectation of cash flows. Under the accounting model for PCI loans, the excess of cash flows expected to be collected over the carrying amount of the loans, referred to as the “accretable yield”, is accreted into interest income over the life of the loans in each pool using the level yield method. Accordingly, PCI loans are not subject to classification as non-accrual Subsequent to acquisition, actual cash collections are monitored relative to management’s expectations and revised cash flow forecasts are prepared, as warranted. These revised forecasts involve updates, as necessary, of the key assumptions and estimates used in the initial estimate of fair value. Generally speaking, expected cash flows are affected by: • Changes in the expected principal and interest payments over the estimated life • Changes in prepayment assumptions • Changes in interest rate indices for variable rate loans A decrease in expected cash flows in subsequent periods may indicate that the loan pool is impaired, which

Stockholders' Equity

Stockholders' Equity	9 Months Ended
	Sep. 30, 2018
Federal Home Loan Banks [Abstract]
Stockholders' Equity	NOTE 4. STOCKHOLDERS’ EQUITY Preferred Stock and Common Stock People’s United is authorized to issue (i) 50.0 million shares of preferred stock, par value of $0.01 per share, of which 10.0 million shares were outstanding at both September 30, 2018 and December 31, 2017, and (ii) 1.95 billion shares of common stock, par value of $0.01 per share, of which 437.7 million shares and 435.6 million shares were issued at September 30, 2018 and December 31, 2017, respectively. Treasury Stock Treasury stock includes (i) common stock repurchased by People’s United, either directly or through agents, in the open market at prices and terms satisfactory to management in connection with stock repurchases authorized by its Board of Directors (86.4 million shares at both September 30, 2018 and December 31, 2017) and (ii) common stock purchased in the open market by a trustee with funds provided by People’s United and originally intended for awards under the People’s United Financial, Inc. 2007 Recognition and Retention Plan (the “RRP”) (2.6 million shares at both September 30, 2018 and December 31, 2017). Following shareholder approval of the People’s United Financial, Inc. 2014 Long-Term Incentive Plan in 2014, no new awards may be granted under the RRP. Comprehensive Income Comprehensive income represents the sum of net income and items of “other comprehensive income or loss,” including (on an after-tax available-for-sale; held-to-maturity; The following is a summary of the changes in the components of AOCL, which are included in People’s United’s stockholders’ equity on an after-tax (in millions) Pension Net Unrealized Available-for-Sale Net Unrealized Held-to-Maturity Net Unrealized Total Balance at December 31, 2017 $ (144.1 ) $ (21.6 ) $ (15.1 ) $ (0.9 ) $ (181.7 ) Other comprehensive income (loss) before reclassifications — (56.2 ) — (2.4 ) (58.6 ) Amounts reclassified from AOCL (1) 4.5 (0.1 ) 2.3 0.1 6.8 Current period other comprehensive 4.5 (56.3 ) 2.3 (2.3 ) (51.8 ) Transition adjustments related to adoption of new accounting standards (2) (30.0 ) (3.9 ) (3.2 ) (0.2 ) (37.3 ) Balance at September 30, 2018 $ (169.6 ) $ (81.8 ) $ (16.0 ) $ (3.4 ) $ (270.8 ) (in millions) Pension and Net Unrealized Available-for-Sale Net Unrealized Held-to-Maturity Net Unrealized Total Balance at December 31, 2016 $ (145.6 ) $ (32.3 ) $ (17.4 ) $ 0.3 $ (195.0 ) Other comprehensive income before reclassifications — 11.1 — 0.3 11.4 Amounts reclassified from AOCL (1) 2.8 9.9 1.6 (0.4 ) 13.9 Current period other comprehensive 2.8 21.0 1.6 (0.1 ) 25.3 Balance at September 30, 2017 $ (142.8 ) $ (11.3 ) $ (15.8 ) $ 0.2 $ (169.7 ) (1) See the following table for details about these reclassifications. (2) See Note 13. The following is a summary of the amounts reclassified from AOCL: Amounts Reclassified from AOCL Affected Line Item Three Months Ended Nine Months Ended (in millions) 2018 2017 2018 2017 Details about components of AOCL: Amortization of pension and other postretirement plans items: Net actuarial loss $ (2.0 ) $ (1.7 ) $ (6.2 ) $ (5.1 ) (1) Prior service credit — 0.2 0.2 0.6 (1) (2.0 ) (1.5 ) (6.0 ) (4.5 ) Income before income tax expense 0.5 0.6 1.5 1.7 Income tax expense (1.5 ) (0.9 ) (4.5 ) (2.8 ) Net income Reclassification adjustment for net realized gains (losses) on debt securities available-for-sale — — 0.1 (15.6 ) Income before income tax expense (2) — — — 5.7 Income tax expense — — 0.1 (9.9 ) Net income Amortization of unrealized losses on debt securities transferred to held-to-maturity (0.9 ) (0.9 ) (3.0 ) (2.6 ) Income before income tax expense (3) 0.2 0.3 0.7 1.0 Income tax expense (0.7 ) (0.6 ) (2.3 ) (1.6 ) Net income Amortization of unrealized gains and losses on cash flow hedges: Interest rate swaps (0.3 ) 0.3 (0.3 ) 0.6 (4) Interest rate locks 0.1 0.1 0.1 0.1 (4) (0.2 ) 0.4 (0.2 ) 0.7 Income before income tax expense 0.1 (0.2 ) 0.1 (0.3 ) Income tax expense (0.1 ) 0.2 (0.1 ) 0.4 Net income Total reclassifications for the period $ (2.3 ) $ (1.3 ) $ (6.8 ) $ (13.9 ) (1) Included in the computation of net periodic benefit income (expense) reflected in other non-interest (2) Included in other non-interest (3) Included in interest and dividend income - securities. (4) Included in interest expense - notes and debentures.

Earnings Per Common Share

Earnings Per Common Share	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Earnings Per Common Share	NOTE 5. EARNINGS PER COMMON SHARE The following is an analysis of People’s United’s basic and diluted earnings per common share (“EPS”), reflecting the application of the two-class Three Months Ended Nine Months Ended (in millions, except per common share data) 2018 2017 2018 2017 Net income available to common shareholders $ 113.5 $ 87.3 $ 324.6 $ 220.4 Dividends paid on and undistributed earnings allocated to participating securities — (0.1 ) (0.2 ) (0.4 ) Earnings attributable to common shareholders $ 113.5 $ 87.2 $ 324.4 $ 220.0 Weighted average common shares outstanding for basic EPS 341.4 336.9 340.6 327.6 Effect of dilutive equity-based awards 3.6 1.9 3.9 2.0 Weighted average common shares and common-equivalent shares for diluted EPS 345.0 338.8 344.5 329.6 EPS: Basic $ 0.33 $ 0.26 $ 0.95 $ 0.67 Diluted 0.33 0.26 0.94 0.67 Unvested share-based payment awards, which include the right to receive non-forfeitable two-class two-class All unallocated Employee Stock Ownership Plan (“ESOP”) common shares and all common shares accounted for as treasury shares have been excluded from the calculation of basic and diluted EPS. Anti-dilutive equity-based awards totaling 6.8 million shares for both the three and nine months ended September 30, 2018, and 10.2 million shares and 10.1 million shares for the three and nine months ended September 30, 2017, respectively, have also been excluded from the calculation of diluted EPS.

Goodwill and Other Acquisition-

Goodwill and Other Acquisition-Related Intangible Assets	9 Months Ended
	Sep. 30, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Other Acquisition-Related Intangible Assets	NOTE 6. GOODWILL AND OTHER ACQUISITION-RELATED INTANGIBLE ASSETS Changes in the carrying amounts of People’s United’s goodwill for the nine months ended September 30, 2018 and 2017 are summarized as follows: Operating Segment (in millions) Commercial Retail Wealth Total Balance at December 31, 2017 $ 1,600.3 $ 720.1 $ 91.0 $ 2,411.4 Acquisition of Vend Lease 23.8 — — 23.8 Balance at September 30, 2018 $ 1,624.1 $ 720.1 $ 91.0 $ 2,435.2 Operating Segment (in millions) Commercial Retail Wealth Total Balance at December 31, 2016 $ 1,222.1 $ 679.6 $ 91.0 $ 1,992.7 Acquisition of: Suffolk Bancorp 229.8 40.5 — 270.3 LEAF 148.4 — — 148.4 Balance at September 30, 2017 $ 1,600.3 $ 720.1 $ 91.0 $ 2,411.4 Recent acquisitions have been undertaken with the objective of expanding the Company’s business, both geographically and through product offerings, as well as realizing synergies and economies of scale by combining with the acquired entities. For these reasons, a market-based premium was paid for the acquired entities which, in turn, resulted in the recognition of goodwill, representing the excess of the respective purchase prices over the estimated fair value of the net assets acquired. All of People’s United’s tax deductible goodwill was created in transactions in which the Company purchased the assets of the target (as opposed to purchasing the issued and outstanding stock of the target). At September 30, 2018 and December 31, 2017, tax deductible goodwill totaled $91.5 million and $72.3 million, respectively. People’s United’s other acquisition-related intangible assets totaled $133.7 million and $148.6 million at September 30, 2018 and December 31, 2017, respectively. At September 30, 2018, the carrying amounts of other acquisition-related intangible assets were as follows: trade name ($60.9 million); client relationship intangible ($20.5 million); core deposit intangible ($18.1 million); trust relationship intangible ($12.1 million); insurance relationship intangible ($4.7 million); favorable lease agreement ($0.5 million); non-compete Amortization expense of other acquisition-related intangible assets totaled $4.9 million and $7.9 million for the three months ended September 30, 2018 and 2017, respectively, and $14.9 million and $22.1 million for the nine months ended September 30, 2018 and 2017, respectively. Scheduled amortization expense attributable to other acquisition-related intangible assets for the full-year of 2018 and each of the next five years is as follows: $19.9 million in 2018; $18.4 million in 2019; $16.8 million in 2020; $15.0 million in 2021; $13.5 million in 2022; and $8.5 million in 2023. There were no impairment losses relating to goodwill or other acquisition-related intangible assets recorded during the nine months ended September 30, 2018 and 2017.

Employee Benefit Plans

Employee Benefit Plans	9 Months Ended
	Sep. 30, 2018
Retirement Benefits [Abstract]
Employee Benefit Plans	NOTE 7. EMPLOYEE BENEFIT PLANS People’s United Employee Pension and Other Postretirement Plans People’s United maintains a qualified noncontributory defined benefit pension plan (the “People’s Qualified Plan”) that covers substantially all full-time and part-time employees who (i) meet certain age and length of service requirements and (ii) were employed by the Bank prior to August 14, 2006. Benefits are based upon the employee’s years of credited service and either the average compensation for the last five years or the average compensation for the five consecutive years of the last ten years that produce the highest average. New employees of the Bank starting on or after August 14, 2006 are not eligible to participate in the People’s Qualified Plan. Instead, the Bank makes contributions on behalf of these employees to a qualified defined contribution plan in an annual amount equal to 3% of the employee’s eligible compensation. Employee participation in this plan is restricted to employees who (i) are at least 18 years of age and (ii) worked at least 1,000 hours in a year. Both full-time and part-time employees are eligible to participate as long as they meet these requirements. In July 2011, the Bank amended the People’s Qualified Plan to “freeze”, effective December 31, 2011, the accrual of pension benefits for People’s Qualified Plan participants. As such, participants will not earn any additional benefits after that date. Instead, effective January 1, 2012, the Bank began making contributions on behalf of these participants to a qualified defined contribution plan in an annual amount equal to 3% of the employee’s eligible compensation. In addition to the People’s Qualified Plan, People’s United continues to maintain a qualified defined benefit pension plan that covers former Chittenden employees who meet certain eligibility requirements (the “Chittenden Qualified Plan”). Effective December 31, 2005, accrued benefits were frozen based on participants’ then-current service and pay levels. Interest continues to be credited on undistributed balances at a crediting rate specified by the Chittenden Qualified Plan. During April 2010, participants who were in payment status as of April 1, 2010, or whose accrued benefit as of that date was scheduled to be paid in the form of an annuity commencing May 1, 2010 based upon elections made by April 15, 2010, were transferred into the People’s Qualified Plan. People’s United also continues to maintain a qualified defined benefit pension plan that covers former Suffolk Bancorp employees who meet certain eligibility requirements (the “Suffolk Qualified Plan”). Effective December 31, 2012, accrued benefits were frozen based on participants’ then-current service and pay levels. Interest continues to be credited on undistributed balances at a crediting rate specified by the Suffolk Qualified Plan. People’s United’s funding policy is to contribute the amounts required by applicable regulations, although additional amounts may be contributed from time to time. In February 2018, People’s United made voluntary employer contributions of $40 million to the People’s Qualified Plan and $10 million to the Chittenden Qualified Plan (none to the Suffolk Qualified Plan) in response to tax reform. People’s United also maintains (i) unfunded, nonqualified supplemental plans to provide retirement benefits to certain senior officers (the “Supplemental Plans”) and (ii) an unfunded plan that provides retirees with optional medical, dental and life insurance benefits (the “Other Postretirement Plan”). People’s United accrues the cost of these postretirement benefits over the employees’ years of service to the date of their eligibility for such benefit. Components of net periodic benefit (income) expense and other amounts recognized in other comprehensive income (loss) for the People’s Qualified Plan, the Chittenden Qualified Plan and the Supplemental Plans (together the “Pension Plans”) and the Other Postretirement Plan are as follows: Pension Plans Other Three months ended September 30 (in millions) 2018 2017 2018 2017 Net periodic benefit (income) expense: Interest cost $ 4.9 $ 4.9 $ 0.2 $ 0.2 Expected return on plan assets (10.9 ) (9.6 ) — — Recognized net actuarial loss 2.0 1.6 — 0.1 Recognized prior service credit — (0.2 ) — — Settlements (0.5 ) 1.1 — — Net periodic benefit (income) expense (1) $ (4.5 ) $ (2.2 ) $ 0.2 $ 0.3 Pension Plans Other Nine months ended September 30 (in millions) 2018 2017 2018 2017 Net periodic benefit (income) expense: Interest cost $ 14.6 $ 14.3 $ 0.6 $ 0.6 Expected return on plan assets (32.7 ) (28.3 ) — — Recognized net actuarial loss 6.0 4.9 0.2 0.2 Recognized prior service credit (0.2 ) (0.6 ) — — Settlements 0.4 2.6 — — Net periodic benefit (income) expense (1) (11.9 ) (7.1 ) 0.8 0.8 Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss): Net actuarial loss (6.0 ) (4.9 ) (0.2 ) (0.2 ) Prior service credit 0.2 0.6 — — Total pre-tax (5.8 ) (4.3 ) (0.2 ) (0.2 ) Total recognized in net periodic benefit (income) expense and other comprehensive income (loss) $ (17.7 ) $ (11.4 ) $ 0.6 $ 0.6 (1) As discussed in Note 13, amounts are included in other non-interest Employee Stock Ownership Plan In April 2007, People’s United established an ESOP. At that time, People’s United loaned the ESOP $216.8 million to purchase 10,453,575 shares of People’s United common stock in the open market. In order for the ESOP to repay the loan, People’s United expects to make annual cash contributions of approximately $18.8 million until 2036. Such cash contributions may be reduced by the cash dividends paid on unallocated ESOP shares, which, for the nine months ended September 30, 2018, totaled $3.5 million. At September 30, 2018, the loan balance totaled $180.8 million. Employee participation in this plan is restricted to those employees who (i) are at least 18 years of age and (ii) worked at least 1,000 hours within 12 months of their hire date or any plan year (January 1 to December 31) after their date of hire. Employees meeting the aforementioned eligibility criteria during the plan year must continue to be employed as of the last day of the plan year in order to receive an allocation of shares for that plan year. Shares of People’s United common stock are held by the ESOP and allocated to eligible participants annually based upon a percentage of each participant’s eligible compensation. Since the ESOP was established, a total of 4,094,316 shares of People’s United common stock have been allocated or committed to be released to participants’ accounts. At September 30, 2018, 6,359,259 shares of People’s United common stock, with a fair value of $108.9 million at that date, have not been allocated or committed to be released. Compensation expense related to the ESOP is recognized at an amount equal to the number of common shares committed to be released by the ESOP for allocation to participants’ accounts multiplied by the average fair value of People’s United’s common stock during the reporting period. The difference between the fair value of the shares of People’s United’s common stock committed to be released and the cost of those common shares is recorded as a credit to additional paid-in paid-in

Legal Proceedings

Legal Proceedings	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Legal Proceedings	NOTE 8. LEGAL PROCEEDINGS In the normal course of business, People’s United is subject to various legal proceedings. Management has discussed with legal counsel the nature of these legal proceedings and, based on the advice of counsel and the information currently available, believes that the eventual outcome of these legal proceedings will not have a material adverse effect on People’s United’s financial condition, results of operations or liquidity.

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2018
Segment Reporting [Abstract]
Segment Information	NOTE 9. SEGMENT INFORMATION See “Segment Results” included in Item 2 for segment information for the three and nine months ended September 30, 2018 and 2017.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	NOTE 10. FAIR VALUE MEASUREMENTS Accounting standards related to fair value measurements define fair value, provide a framework for measuring fair value and establish related disclosure requirements. Broadly, fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants at the measurement date. Accordingly, an “exit price” approach is required in determining fair value. In support of this principle, a fair value hierarchy has been established that prioritizes the inputs used to measure fair value, requiring entities to maximize the use of market or observable inputs (as more reliable measures) and minimize the use of unobservable inputs. Observable inputs reflect market data obtained from independent sources, while unobservable inputs generally require significant management judgment. The three levels within the fair value hierarchy are as follows: • Level 1 – Unadjusted quoted market prices for identical assets or liabilities in active markets that the entity has the ability to access at the measurement date (such as active exchange-traded equity securities or mutual funds and certain U.S. and government agency debt securities). • Level 2 – Observable inputs other than quoted prices included in Level 1, such as: • quoted prices for similar assets or liabilities in active markets (such as U.S. agency and GSE mortgage-backed securities); • quoted prices for identical or similar assets or liabilities in less active markets (such as certain U.S. and government agency debt securities, and corporate and municipal debt securities that trade infrequently); and • other inputs that (i) are observable for substantially the full term of the asset or liability (e.g. interest rates, yield curves, prepayment speeds, default rates, etc.) or (ii) can be corroborated by observable market data (such as interest rate and currency derivatives and certain other securities). • Level 3 – Valuation techniques that require unobservable inputs that are supported by little or no market activity and are significant to the fair value measurement of the asset or liability (such as pricing models, discounted cash flow methodologies and similar techniques that typically reflect management’s own estimates of the assumptions a market participant would use in pricing the asset or liability). People’s United maintains policies and procedures to value assets and liabilities using the most relevant data available. Described below are the valuation methodologies used by People’s United and the resulting fair values for those financial instruments measured at fair value on both a recurring and a non-recurring non-recurring Recurring Fair Value Measurements Trading Debt Securities, Equity Securities and Debt Securities Available-For-Sale When available, People’s United uses quoted market prices for identical securities received from an independent, nationally-recognized, third-party pricing service (as discussed further below) to determine the fair value of investment securities such as U.S. Treasury and agency securities and equity securities that are included in Level 1. When quoted market prices for identical securities are unavailable, People’s United uses prices provided by the independent pricing service based on recent trading activity and other observable information including, but not limited to, market interest rate curves, referenced credit spreads and estimated prepayment rates where applicable. These investments include certain U.S. and government agency debt securities, corporate and municipal debt securities, and GSE mortgage-backed securities, all of which are included in Level 2. The Company’s available-for-sale available-for-sale 10- 15-year Changes in the prices obtained from the pricing service are analyzed from month to month, taking into consideration changes in market conditions including changes in mortgage spreads, changes in U.S. Treasury security yields and changes in generic pricing of securities with similar duration. As a further point of validation, the Company generates its own month-end Other Assets As discussed in Note 7, certain unfunded, nonqualified supplemental plans have been established to provide retirement benefits to certain senior officers. People’s United has funded two trusts to provide benefit payments to the extent such benefits are not paid directly by People’s United, the assets of which are included in other assets in the Consolidated Statements of Condition. When available, People’s United determines the fair value of the trust assets using quoted market prices for identical securities received from a third-party nationally recognized pricing service. Derivatives People’s United values its derivatives using internal models that are based on market or observable inputs including interest rate curves and forward/spot prices for selected currencies. Derivative assets and liabilities included in Level 2 represent interest rate swaps and caps, foreign exchange contracts, risk participation agreements, forward commitments to sell residential mortgage loans and interest rate-lock commitments on residential mortgage loans. The following tables summarize People’s United’s financial instruments that are measured at fair value on a recurring basis: Fair Value Measurements Using As of September 30, 2018 (in millions) Level 1 Level 2 Level 3 Total Financial assets: Trading debt securities: U.S. Treasury $ 8.3 $ — $ — $ 8.3 Debt securities available-for-sale: U.S. Treasury and agency 652.1 — — 652.1 GSE mortgage-backed securities — 2,660.0 — 2,660.0 Equity securities 8.9 — — 8.9 Other assets: Exchange-traded funds 39.1 — — 39.1 Mutual funds 3.1 — — 3.1 Fixed income securities — 0.3 — 0.3 Interest rate swaps — 45.9 — 45.9 Interest rate caps — 3.6 — 3.6 Foreign exchange contracts — 0.6 — 0.6 Forward commitments to sell residential mortgage loans — 0.2 — 0.2 Total $ 711.5 $ 2,710.6 $ — $ 3,422.1 Financial liabilities: Interest rate swaps $ — $ 207.4 $ — $ 207.4 Interest rate caps — 3.6 — 3.6 Risk participation agreements (1) — — — — Foreign exchange contracts — 0.4 — 0.4 Interest rate-lock commitments on residential mortgage loans — 0.2 — 0.2 Total $ — $ 211.6 $ — $ 211.6 Fair Value Measurements Using As of December 31, 2017 (in millions) Level 1 Level 2 Level 3 Total Financial assets: Trading debt securities: U.S. Treasury $ 8.2 $ — $ — $ 8.2 Debt securities available-for-sale: U.S. Treasury and agency 668.8 — — 668.8 GSE mortgage-backed securities — 2,456.5 — 2,456.5 Equity securities 8.7 — — 8.7 Other assets: Exchange-traded funds 36.5 — — 36.5 Mutual funds 3.5 — — 3.5 Fixed income securities — 1.3 — 1.3 Interest rate swaps — 74.8 — 74.8 Interest rate caps — 2.8 — 2.8 Foreign exchange contracts — 0.1 — 0.1 Forward commitments to sell residential mortgage loans — 0.2 — 0.2 Total $ 725.7 $ 2,535.7 $ — $ 3,261.4 Financial liabilities: Interest rate swaps $ — $ 84.9 $ — $ 84.9 Interest rate caps — 2.8 — 2.8 Risk participation agreements (1) — — — — Foreign exchange contracts — 0.4 — 0.4 Interest rate-lock commitments on residential mortgage loans — 0.2 — 0.2 Total $ — $ 88.3 $ — $ 88.3 (1) At both September 30, 2018 and December 31, 2017, the fair value of risk participation agreements totaled less than $0.1 million (see Note 11). There were no transfers into or out of the Level 1 or Level 2 categories during the nine months ended September 30, 2018 or 2017. Non-Recurring Loans Held-for-Sale Residential mortgage loans held-for-sale non-recurring Impaired Loans Loan impairment is deemed to exist when full repayment of principal and interest according to the contractual terms of the loan is no longer probable. Impaired loans are reported based on one of three measures: (i) the present value of expected future cash flows discounted at the loan’s original effective interest rate; (ii) the loan’s observable market price; or (iii) the fair value of the collateral (less estimated cost to sell) if the loan is collateral dependent. Accordingly, certain impaired loans may be subject to measurement at fair value on a non-recurring People’s United has estimated the fair values of these assets using Level 3 inputs, such as discounted cash flows based on inputs that are largely unobservable and, instead, reflect management’s own estimates of the assumptions a market participant would use in pricing such loans and/or the fair value of collateral based on independent third-party appraisals for collateral-dependent loans. Such appraisals are based on the market and/or income approach to value and are subject to a discount (to reflect estimated cost to sell) that generally approximates 10%. REO and Repossessed Assets REO and repossessed assets are recorded at the lower of cost or fair value, less estimated selling costs, and are therefore measured at fair value on a non-recurring held-for-sale The following tables summarize People’s United’s assets that are measured at fair value on a non-recurring Fair Value Measurements Using As of September 30, 2018 (in millions) Level 1 Level 2 Level 3 Total Loans held-for-sale $ — $ 15.2 $ — $ 15.2 Impaired loans (2) — — 54.1 54.1 REO and repossessed assets (3) — — 15.1 15.1 Total $ — $ 15.2 $ 69.2 $ 84.4 Fair Value Measurements Using As of December 31, 2017 (in millions) Level 1 Level 2 Level 3 Total Loans held-for-sale $ — $ 16.6 $ — $ 16.6 Impaired loans (2) — — 59.6 59.6 REO and repossessed assets (3) — — 19.4 19.4 Total $ — $ 16.6 $ 79.0 $ 95.6 (1) Consists of residential mortgage loans; no fair value adjustments were recorded for the nine months ended September 30, 2018 and 2017. (2) Represents the recorded investment in originated impaired loans with a related allowance for loan losses measured in accordance with applicable accounting guidance. The total consists of $42.0 million of Commercial loans and $12.1 million of Retail loans at September 30, 2018. The provision for loan losses on impaired loans totaled $7.7 million and $4.1 million for the nine months ended September 30, 2018 and 2017, respectively. (3) Represents: (i) $8.7 million of commercial REO; (ii) $4.4 million of residential REO; and (iii) $2.0 million of repossessed assets at September 30, 2018. Charge-offs to the allowance for loan losses related to loans that were transferred to REO or repossessed assets totaled $1.0 million and $2.2 million for the nine months ended September 30, 2018 and 2017, respectively. Write downs and net loss on sale of foreclosed/repossessed assets charged to non-interest Financial Assets and Financial Liabilities Not Measured at Fair Value The following tables summarize the carrying amounts, estimated fair values and placement in the fair value hierarchy of People’s United’s financial instruments that are not measured at fair value either on a recurring or non-recurring Estimated Fair Value Carrying Measurements Using As of September 30, 2018 (in millions) Level 1 Level 2 Level 3 Total Financial assets: Cash and due from banks $ 410.5 $ 410.5 $ — $ — $ 410.5 Short-term investments 127.5 — 127.5 — 127.5 Debt securities held-to-maturity 3,742.9 — 3,665.3 1.5 3,666.8 FHLB and FRB stock 312.4 — 312.4 — 312.4 Total loans, net (1) 31,907.2 — 6,582.7 24,805.5 31,388.2 Financial liabilities: Time deposits 6,035.9 — 5,997.6 — 5,997.6 Other deposits 27,174.3 — 27,174.3 — 27,174.3 FHLB advances 2,369.7 — 2,370.2 — 2,370.2 Federal funds purchased 735.0 — 735.0 — 735.0 Customer repurchase agreements 261.3 — 261.3 — 261.3 Other borrowings 26.0 — 26.0 — 26.0 Notes and debentures 885.6 — 892.5 — 892.5 (1) Excludes impaired loans totaling $54.1 million measured at fair value on a non-recurring Estimated Fair Value Carrying Measurements Using As of December 31, 2017 (in millions) Level 1 Level 2 Level 3 Total Financial assets: Cash and due from banks $ 505.1 $ 505.1 $ — $ — $ 505.1 Short-term investments 377.5 — 377.5 — 377.5 Debt securities held-to-maturity 3,588.1 — 3,632.2 1.5 3,633.7 FHLB and FRB stock 312.3 — 312.3 — 312.3 Total loans, net (1) 32,281.3 — 6,632.2 25,495.3 32,127.5 Financial liabilities: Time deposits 5,454.3 — 5,441.1 — 5,441.1 Other deposits 27,602.0 — 27,602.0 — 27,602.0 FHLB advances 2,774.4 — 2,775.3 — 2,775.3 Federal funds purchased 820.0 — 820.0 — 820.0 Customer repurchase agreements 301.6 — 301.6 — 301.6 Other borrowings 207.8 — 207.2 — 207.2 Notes and debentures 901.6 — 910.1 — 910.1 (1) Excludes impaired loans totaling $59.6 million measured at fair value on a non-recurring

Derivative Financial Instrument

Derivative Financial Instruments and Hedging Activities	9 Months Ended
	Sep. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Financial Instruments and Hedging Activities	NOTE 11. DERIVATIVE FINANCIAL INSTRUMENTS AND HEDGING ACTIVITIES People’s United uses derivative financial instruments as components of its market risk management (principally to manage interest rate risk (“IRR”)). Certain other derivatives are entered into in connection with transactions with commercial customers. Derivatives are not used for speculative purposes. All derivatives are recognized as either assets or liabilities in the Consolidated Statements of Condition, reported at fair value and presented on a gross basis. Until a derivative is settled, a favorable change in fair value results in an unrealized gain that is recognized as an asset, while an unfavorable change in fair value results in an unrealized loss that is recognized as a liability. The Company generally applies hedge accounting to its derivatives used for market risk management purposes. Hedge accounting is permitted only if specific criteria are met, including a requirement that a highly effective relationship exist between the derivative instrument and the hedged item, both at inception of the hedge and on an ongoing basis. The hedge accounting method depends upon whether the derivative instrument is classified as a fair value hedge (i.e. hedging an exposure related to a recognized asset or liability, or a firm commitment) or a cash flow hedge (i.e. hedging an exposure related to the variability of future cash flows associated with a recognized asset or liability, or a forecasted transaction). Changes in the fair value of effective fair value hedges are recognized in current earnings (with the change in fair value of the hedged asset or liability also recorded in earnings). Changes in the fair value of effective cash flow hedges are recognized in other comprehensive income (loss) until earnings are affected by the variability in cash flows of the designated hedged item. Ineffective portions of hedge results are recognized in current earnings. Changes in the fair value of derivatives for which hedge accounting is not applied are recognized in current earnings. People’s United formally documents at inception all relationships between the derivative instruments and the hedged items, as well as its risk management objectives and strategies for undertaking the hedge transactions. This process includes linking all derivatives that are designated as hedges to specific assets and liabilities, or to specific firm commitments or forecasted transactions. People’s United also formally assesses, both at inception of the hedge and on an ongoing basis, whether the derivatives that are used in hedging transactions are highly effective in offsetting changes in the fair values or cash flows of the hedged items. If it is determined that a derivative is not highly effective or has ceased to be a highly effective hedge, People’s United would discontinue hedge accounting prospectively. Gains or losses resulting from the termination of a derivative accounted for as a cash flow hedge remain in AOCL and are amortized to earnings over the remaining period of the former hedging relationship, provided the hedged item continues to be outstanding or it is probable the forecasted transaction will occur. People’s United uses the dollar offset method, regression analysis and scenario analysis to assess hedge effectiveness at inception and on an ongoing basis. Such methods are chosen based on the nature of the hedge strategy and are used consistently throughout the life of the hedging relationship. Certain derivative financial instruments are offered to commercial customers to assist them in meeting their financing and investing objectives and for their risk management purposes. These derivative financial instruments consist primarily of interest rate swaps and caps, but also include foreign exchange contracts. The interest rate and foreign exchange risks associated with customer interest rate swaps and caps and foreign exchange contracts are mitigated by entering into similar derivatives having essentially offsetting terms with institutional counterparties. Interest rate-lock commitments extended to borrowers relate to the origination of residential mortgage loans. To mitigate the IRR inherent in these commitments, People’s United enters into mandatory delivery and best efforts contracts to sell adjustable-rate and fixed-rate residential mortgage loans (servicing released). Forward commitments to sell and interest rate-lock commitments on residential mortgage loans are considered derivatives and their respective estimated fair values are adjusted based on changes in interest rates. Changes in the fair value of derivatives for which hedge accounting is not applied are recognized in current earnings, including customer derivatives, interest-rate lock commitments and forward sale commitments. By using derivatives, People’s United is exposed to credit risk to the extent that counterparties to the derivative contracts do not perform as required. Should a counterparty fail to perform under the terms of a derivative contract, the Company’s counterparty credit risk is equal to the amount reported as a derivative asset in the Consolidated Statements of Condition. In accordance with the Company’s balance sheet offsetting policy (see Note 12), amounts reported as derivative assets represent derivative contracts in a gain position, without consideration for derivative contracts in a loss position with the same counterparty (to the extent subject to master netting arrangements) and posted collateral. People’s United seeks to minimize counterparty credit risk through credit approvals, limits, monitoring procedures, execution of master netting arrangements and obtaining collateral, where appropriate. Counterparties to People’s United’s derivatives include major financial institutions and exchanges that undergo comprehensive and periodic internal credit analysis as well as maintain investment grade credit ratings from the major credit rating agencies. As such, management believes the risk of incurring credit losses on derivative contracts with those counterparties is remote and losses, if any, would be immaterial. Certain of People’s United’s derivative contracts contain provisions establishing collateral requirements (subject to minimum collateral posting thresholds) based on the Company’s external credit rating. If the Company’s senior unsecured debt rating were to fall below the level generally recognized as investment grade, the counterparties to such derivative contracts could require additional collateral on those derivative transactions in a net liability position (after considering the effect of master netting arrangements and posted collateral). There were no derivative instruments with such credit-related contingent features in a net liability position at September 30, 2018. The following sections further discuss each class of derivative financial instrument used by People’s United, including management’s principal objectives and risk management strategies. Interest Rate Swaps People’s United may, from time to time, enter into interest rate swaps that are used to manage IRR associated with certain interest-earning assets and interest-bearing liabilities. The Bank has entered into pay floating/receive fixed interest rate swaps to reduce its IRR exposure to the variability in interest cash flows on certain floating-rate commercial loans. The Bank has agreed with the swap counterparties to exchange, at specified intervals, the difference between fixed-rate and floating-rate interest amounts calculated based on notional amounts totaling $210 million. The floating-rate interest payments made under the swaps are calculated using the same floating rate received on the commercial loans. The swaps effectively convert the floating-rate one-month The Bank has entered into a pay floating/receive fixed interest rate swap to hedge the change in fair value due to changes in interest rates of the Bank’s $400 million subordinated notes. The Bank has agreed with the swap counterparty to exchange, at specified intervals, the difference between fixed-rate and floating-rate interest amounts calculated based on a notional amount of $375 million. The fixed-rate interest payments received on the swap will essentially offset the fixed-rate interest payments made on these notes, notwithstanding the notional difference between these notes and the swap. The floating-rate interest amounts paid under the swap are calculated based on three-month LIBOR plus 126.5 basis points. The swap effectively converts the fixed-rate subordinated notes to a floating-rate liability. This swap is accounted for as a fair value hedge. Customer Derivatives People’s United enters into interest rate swaps and caps with certain of its commercial customers. In order to minimize its risk, these customer interest rate swaps (pay floating/receive fixed) and caps have been offset with essentially matching interest rate swaps (pay fixed/receive floating) and caps with People’s United’s institutional counterparties. Hedge accounting has not been applied for these derivatives. Accordingly, changes in the fair value of all such interest rate swaps and caps are recognized in current earnings. Foreign Exchange Contracts Foreign exchange contracts are commitments to buy or sell foreign currency on a future date at a contractual price. People’s United uses these instruments on a limited basis to (i) eliminate its exposure to fluctuations in currency exchange rates on certain of its commercial loans that are denominated in foreign currencies and (ii) provide foreign exchange contracts on behalf of commercial customers within credit exposure limits. Gains and losses on foreign exchange contracts substantially offset the translation gains and losses on the related loans. Risk Participation Agreements People’s United enters into risk participation agreements under which it may either assume or sell credit risk associated with a borrower’s performance under certain interest rate derivative contracts. In those instances in which People’s United has assumed credit risk, it is not a party to the derivative contract and has entered into the risk participation agreement because it is also a party to the related loan agreement with the borrower. In those instances in which People’s United has sold credit risk, it is a party to the derivative contract and has entered into the risk participation agreement because it sold a portion of the related loan. People’s United manages its credit risk under risk participation agreements by monitoring the creditworthiness of the borrower, based on its normal credit review process. The notional amounts of the risk participation agreements reflect People’s United’s pro-rata Forward Commitments to Sell Residential Mortgage Loans and Related Interest Rate-Lock Commitments People’s United enters into forward commitments to sell adjustable-rate and fixed-rate residential mortgage loans (all to be sold servicing released) in order to reduce the market risk associated with originating loans for sale in the secondary market. In order to fulfill a forward commitment, People’s United delivers originated loans at prices or yields specified by the contract. The risks associated with such contracts arise from the possible inability of counterparties to meet the contract terms or People’s United’s inability to originate the necessary loans. Gains and losses realized on the forward contracts are reported in the Consolidated Statements of Income as a component of the net gains on sales of residential mortgage loans. In the normal course of business, People’s United will commit to an interest rate on a mortgage loan application at the time of application, or anytime thereafter. The risks associated with these interest rate-lock commitments arise if market interest rates change prior to the closing of these loans. Both forward sales commitments and interest rate-lock commitments made to borrowers on held-for-sale Interest Rate Locks In connection with its planned issuance of senior notes in the fourth quarter of 2012, People’s United entered into U.S. Treasury forward interest rate locks (“T-Locks”) 10-year T-Locks net-of-tax ten-year The table below provides a summary of the notional amounts and fair values (presented on a gross basis) of derivatives outstanding: Fair Values (1) Notional Amounts Assets Liabilities (in millions) Type of Sept. 30, Dec. 31, Sept. 30, Dec. 31, Sept. 30, Dec. 31, Derivatives Not Designated as Hedging Instruments: Interest rate swaps: Commercial customers N/A $ 6,576.2 $ 5,769.1 $ 17.5 $ 64.7 $ 199.1 $ 61.2 Institutional counterparties N/A 6,282.0 5,775.9 28.4 10.1 8.3 23.7 Interest rate caps: Commercial customers N/A 313.0 649.2 — — 3.6 2.8 Institutional counterparties N/A 313.0 649.2 3.6 2.8 — — Risk participation agreements (2) N/A 455.4 439.4 — — — — Foreign exchange contracts N/A 99.2 46.5 0.6 0.1 0.4 0.4 Forward commitments to sell N/A 18.9 16.4 0.2 0.2 — — Interest rate-lock commitments on residential mortgage loans N/A 24.0 18.3 — — 0.2 0.2 Total 50.3 77.9 211.6 88.3 Derivatives Designated as Hedging Instruments: Interest rate swaps: Subordinated notes Fair value 375.0 375.0 — — — — Loans Cash flow 210.0 210.0 — — — — Total — — — — Total fair value of derivatives $ 50.3 $ 77.9 $ 211.6 $ 88.3 (1) Assets are recorded in other assets and liabilities are recorded in other liabilities. (2) Fair value totaled less than $0.1 million at both dates. The following table summarizes the impact of People’s United’s derivatives on pre-tax Amount of Pre-Tax Gain (Loss) Amount of Pre-Tax Type of Recognized in Earnings (1) Recognized in AOCL Nine months ended September 30 (in millions) 2018 2017 2018 2017 Derivatives Not Designated as Hedging Instruments: Interest rate swaps: Commercial customers N/A $ (177.1 ) $ 44.2 $ — $ — Institutional counterparties N/A 184.5 (38.4 ) — — Interest rate caps: Commercial customers N/A (0.7 ) 1.2 — — Institutional counterparties N/A 0.8 (0.7 ) — — Foreign exchange contracts N/A 0.6 0.3 — — Risk participation agreements N/A 0.2 0.5 — — Forward commitments to sell residential mortgage loans N/A — 0.1 — — Interest rate-lock commitments on residential mortgage loans N/A — (0.1 ) — — Total 8.3 7.1 — — Derivatives Designated as Hedging Instruments: Interest rate swaps Fair value (1.8 ) 4.5 — — Interest rate swaps Cash flow (0.3 ) 0.6 (3.1 ) 0.4 Interest rate locks Cash flow 0.1 0.1 — — Total (2.0 ) 5.2 (3.1 ) 0.4 Total $ 6.3 $ 12.3 $ (3.1 ) $ 0.4 (1) Amounts recognized in earnings are recorded in interest income, interest expense or other non-interest non-interest

Balance Sheet Offsetting

Balance Sheet Offsetting	9 Months Ended
	Sep. 30, 2018
Text Block [Abstract]
Balance Sheet Offsetting	NOTE 12. BALANCE SHEET OFFSETTING Assets and liabilities relating to certain financial instruments, including derivatives, may be eligible for offset in the Consolidated Statements of Condition and/or subject to enforceable master netting arrangements or similar agreements. People’s United’s derivative transactions with institutional counterparties are generally executed under International Swaps and Derivative Association (“ISDA”) master agreements, which include “right of set-off” The Chicago Mercantile Exchange (“CME”) characterizes variation margin payments for over-the-counter Collateral (generally in the form of marketable debt securities) pledged by counterparties in connection with derivative transactions is not reported in the Consolidated Statements of Condition unless the counterparty defaults. Collateral that has been pledged by People’s United to counterparties continues to be reported in the Consolidated Statements of Condition unless the Company defaults. The following tables provide a gross presentation, the effects of offsetting, and a net presentation of the Company’s financial instruments that are eligible for offset in the Consolidated Statements of Condition. The collateral amounts in these tables are limited to the outstanding balances of the related asset or liability (after netting is applied) and, therefore, instances of overcollateralization are not presented. In the tables below, the Net Amount Presented of the derivative assets and liabilities can be reconciled to the fair value of the Company’s derivative financial instruments in Note 11. The Company’s derivative contracts with commercial customers and customer repurchase agreements are not subject to master netting arrangements and, therefore, have been excluded from the tables below. Gross Gross Net Gross Amounts Not Offset Financial Net As of September 30, 2018 (in millions) Instruments Collateral Amount Financial assets: Interest rate swaps: Counterparty A $ 5.1 $ — $ 5.1 $ (1.0 ) $ (4.1 ) $ — Counterparty B 4.4 — 4.4 (2.7 ) (1.7 ) — Counterparty C 10.3 — 10.3 (1.2 ) (9.1 ) — Counterparty D 6.3 — 6.3 (1.4 ) (4.9 ) — Counterparty E — — — — — — Other counterparties 5.9 — 5.9 (0.1 ) (5.7 ) 0.1 Foreign exchange contracts 0.6 — 0.6 — — 0.6 Total $ 32.6 $ — $ 32.6 $ (6.4 ) $ (25.5 ) $ 0.7 Financial liabilities: Interest rate swaps: Counterparty A $ 1.0 $ — $ 1.0 $ (1.0 ) $ — $ — Counterparty B 2.7 — 2.7 (2.7 ) — — Counterparty C 1.2 — 1.2 (1.2 ) — — Counterparty D 1.4 — 1.4 (1.4 ) — — Counterparty E 1.9 — 1.9 — — 1.9 Other counterparties 0.1 — 0.1 (0.1 ) — — Foreign exchange contracts 0.4 — 0.4 — — 0.4 Total $ 8.7 $ — $ 8.7 $ (6.4 ) $ — $ 2.3 Gross Gross Net Gross Amounts Not Offset Financial Net As of December 31, 2017 (in millions) Instruments Collateral Amount Financial assets: Interest rate swaps: Counterparty A $ 2.6 $ — $ 2.6 $ (2.5 ) $ — $ 0.1 Counterparty B 1.6 — 1.6 (1.6 ) — — Counterparty C 2.6 — 2.6 (2.6 ) — — Counterparty D 3.5 — 3.5 (3.5 ) — — Counterparty E — — — — — — Other counterparties 2.6 — 2.6 (0.2 ) (2.4 ) — Foreign exchange contracts 0.1 — 0.1 — — 0.1 Total $ 13.0 $ — $ 13.0 $ (10.4 ) $ (2.4 ) $ 0.2 Financial liabilities: Interest rate swaps: Counterparty A $ 2.5 $ — $ 2.5 $ (2.5 ) $ — $ — Counterparty B 5.6 — 5.6 (1.6 ) (4.0 ) — Counterparty C 2.8 — 2.8 (2.6 ) (0.2 ) — Counterparty D 4.7 — 4.7 (3.5 ) (1.2 ) — Counterparty E 7.3 — 7.3 — — 7.3 Other counterparties 0.8 — 0.8 (0.2 ) (0.6 ) — Foreign exchange contracts 0.4 — 0.4 — — 0.4 Total $ 24.1 $ — $ 24.1 $ (10.4 ) $ (6.0 ) $ 7.7 Resale and repurchase agreements represent agreements to purchase/sell securities subject to an obligation to resell/repurchase the same or similar securities. People’s United accounts for securities resale agreements as secured lending transactions and securities repurchase agreements as secured borrowings since the transferor maintains effective control over the transferred securities and the transfer meets the other criteria for such accounting. The securities are pledged by the transferor as collateral and the transferee has the right by contract to repledge that collateral provided the same collateral is returned to the transferor upon maturity of the underlying agreement. The fair value of the pledged collateral approximates the recorded amount of the secured loan or borrowing. Decreases in the fair value of the transferred securities below an established threshold require the transferor to provide additional collateral. The following tables show the extent to which assets and liabilities exchanged under resale and repurchase agreements have been offset in the Consolidated Statements of Condition. These agreements: (i) are entered into simultaneously with the same financial institution counterparty; (ii) have the same principal amounts and inception/maturity dates; and (iii) are subject to a master netting arrangement that contains a conditional right of offset upon default. At September 30, 2018 and December 31, 2017, the Company posted as collateral marketable securities with fair values of $463.9 million and $453.8 million, respectively, and, in turn, accepted as collateral marketable securities with fair values of $453.8 million and $461.9 million, respectively. As of September 30, 2018 (in millions) Gross Gross Net Total resale agreements $ 450.0 $ (450.0 ) $ — Total repurchase agreements $ 450.0 $ (450.0 ) $ — As of December 31, 2017 (in millions) Gross Gross Net Total resale agreements $ 450.0 $ (450.0 ) $ — Total repurchase agreements $ 450.0 $ (450.0 ) $ —

New Accounting Standards

New Accounting Standards	9 Months Ended
	Sep. 30, 2018
Accounting Changes and Error Corrections [Abstract]
New Accounting Standards	NOTE 13. NEW ACCOUNTING STANDARDS Standards effective in 2018 Revenue Recognition In May 2014, the Financial Accounting Standards Board (the “FASB”) amended its standards with respect to revenue recognition. The amended guidance serves to replace all current GAAP guidance on this topic and eliminate all industry-specific guidance, providing a unified model to determine when and how revenue is recognized. The underlying principle is that a company should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The amendments also require enhanced disclosures regarding the nature, amount, timing and uncertainty of revenue and cash flows arising from an entity’s contracts with customers. As originally issued, this new guidance, which can be applied retrospectively or through the use of the cumulative effect transition method, was to become effective for fiscal years, and interim periods within those fiscal years, beginning on or after December 15, 2016 (January 1, 2017 for People’s United) and early adoption was not permitted. In July 2015, the FASB approved a one-year The Company’s revenue is comprised of net interest income on financial assets and financial liabilities (approximately 75-80%) non-interest 20-25%). non-interest non-deposit In completing its assessment of those revenue streams within the scope of the guidance, the Company identified one revenue source for which the timing of recognition changes under the new standard. The Company previously recognized revenue for certain insurance brokerage activities, such as installments on agency bill, direct bill and contingent commission revenue, over a period of time either due to the transfer of value to customers or as the remuneration becomes determinable. Under the new guidance, certain of these revenues, as well as certain costs associated with originating such policies, are now substantially recognized on the effective date of the associated policies when control of the policy transfers to the customer. The guidance was adopted on January 1, 2018 using the modified retrospective method and resulted in a cumulative-effect transition adjustment which served to increase opening retained earnings by $0.6 million (net-of- For revenue streams within the scope of the guidance, the Company applies the following steps when recognizing revenue from contracts with customers: (i) identify the contract; (ii) identify the performance obligation; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligation; and (v) recognize revenue when the performance obligation is satisfied. The Company’s contracts with customers are generally short-term in nature, typically due within one year or less, or cancellable by the Company or the customer upon a short notice period. Performance obligations for customer contracts are generally satisfied at a single point in time, typically when the transaction is complete, or over time. For performance obligations satisfied over time, the value of the products/services transferred to the customer are evaluated to determine when, and to what degree, performance obligations have been satisfied. Payments from customers are typically received, and revenue recognized, concurrent with the satisfaction of our performance obligations. In most cases, this occurs within a single financial reporting period. For payments received in advance of the satisfaction of our performance obligations, revenue recognition is deferred until such time the performance obligations have been satisfied. In cases where a payment has not been received despite satisfaction of our performance obligations, an estimate of the amount due is accrued in the period our performance obligations have been satisfied. For contracts with variable components, amounts for which collection is probable are accrued. The following summarizes the Company’s performance obligations for those revenue streams deemed to be within the scope of the guidance: Service charges and fees on deposit accounts Card-based and other non-deposit non-deposit Investment management fees month-end Insurance commissions and fees Brokerage commissions and fees The Company generally acts in a principal capacity, on its own behalf, in the majority of its contracts with customers. In such transactions, revenue and the related costs to provide our services are recognized on a gross basis in the financial statements. In some cases, the Company may act in an agent capacity, deriving revenue by assisting other entities in transactions with our customers. In such transactions, revenue and the related costs to provide our services are recognized on a net basis in the financial statements. The extent of the Company’s activities for which it acts as an agent (and for which the related revenue and expense has been presented on a net basis) is immaterial. Presentation of Deferred Taxes In November 2015, the FASB amended its standards with respect to the presentation of deferred income taxes to eliminate the requirement to separate deferred income tax liabilities and assets into current and noncurrent amounts in a classified statement of condition, thereby simplifying the presentation of deferred income taxes. This amendment, which is being applied prospectively, became effective for People’s United on January 1, 2018 and did not have a significant impact on the Company’s Consolidated Financial Statements. Recognition and Measurement of Financial Instruments In January 2016, the FASB amended its standards to address certain aspects of recognition, presentation and disclosure of financial instruments. The amended guidance (i) requires that equity investments (other than equity method investments) be measured at fair value with changes in fair value recognized in net income and (ii) simplifies the impairment assessment of equity investments without readily determinable fair values by permitting a qualitative assessment to identify impairment. Both FRB-NY This amendment became effective for People’s United on January 1, 2018. The cumulative effect transition method was applied to all outstanding instruments as of the date of adoption, while changes to the accounting for equity investments without readily determinable fair values will be applied prospectively. At December 31, 2017, the Company’s securities portfolio included equity securities with an amortized cost of $9.6 million and a fair value of $8.7 million. Accordingly, upon adoption of the guidance, a cumulative-effect transition adjustment, representing the cumulative unrealized loss (net-of-tax) held-for-investment Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments In August 2016, the FASB amended its standards to address the classification of certain cash receipts and payments within the statement of cash flows. Specifically, the amended guidance addresses the following: (i) debt prepayment or debt extinguishment costs; (ii) settlement of zero-coupon Asset Derecognition and Accounting for Partial Sales of Nonfinancial Assets In February 2017, the FASB amended its standards to clarify the scope of its guidance on derecognition of a nonfinancial asset and provide additional guidance on the definition of in-substance Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost In March 2017, the FASB amended its standards to (i) require that the service cost component of net benefit cost associated with pension and postretirement plans be reported in the same line item in which the related employees’ compensation cost is reported and (ii) specify that only the service cost component is eligible for capitalization. The other components of net benefit cost, which may not be capitalized, are to be presented separately. This amendment, which is required to be applied retrospectively, became effective for People’s United on January 1, 2018. Accordingly, net periodic pension and postretirement benefit income (a result of the expected return on plan assets exceeding the sum of the other components) previously reported within compensation and benefits expense is now reported within other non-interest Stock Compensation In July 2017, the FASB amended its standards with respect to share-based payment awards to provide explicit guidance pertaining to the provisions of modification accounting. The amendment clarifies that an entity should not account for the effects of a modification if the award’s fair value, vesting conditions and classification (as either debt or equity) are the same immediately before and after the modification. This amendment, which is being applied prospectively to awards modified on or after the adoption date, became effective for People’s United on January 1, 2018 and did not have a significant impact on the Company’s Consolidated Financial Statements. Standards effective in 2019 Accounting for Leases In February 2016, the FASB amended its standards with respect to the accounting for leases. The amended guidance serves to replace all current GAAP guidance on this topic and requires that an operating lease be recognized on the statement of condition as a “right-of-use” asset along with a corresponding liability representing the rent obligation. Key aspects of current lessor accounting generally remain unchanged from existing guidance, although the definition of eligible initial direct costs (“IDC”) has been amended. Most notably, this standard is expected to result in an increase to assets and liabilities recognized and, therefore, increase risk-weighted assets for regulatory capital purposes. The guidance will become effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018 (January 1, 2019 for People’s United) and, as originally issued, requires the use of the modified retrospective transition approach for existing leases that have not expired before the date of initial application. In July 2018, the FASB issued two targeted improvements to the standard with the objective of reducing the cost and complexity of implementing the guidance. These amendments, which have the same effective date and transition requirements as the new lease standard, serve to (i) introduce an optional transition method allowing entities to recognize a cumulative-effect transition adjustment to the opening balance of retained earnings in the period of adoption rather than in the earliest period presented and (ii) provide a practical expedient whereby lessors can elect, by class of underlying asset, to not separate lease and related non-lease components if certain criteria are met. The Company plans to elect the optional transition method which will result in the modified retrospective approach being applied on January 1, 2019 (as opposed to January 1, 2017). The Company also expects to elect certain transition relief options provided in the standard, including the package of practical expedients. These relief options allow the Company to forego (i) the recognition of right-of-use assets and lease liabilities arising from short-term leases (i.e. leases with terms of twelve months or less) and (ii) a reassessment as to: (a) whether expired or existing contracts are or contain leases; (b) the lease classification for expired or existing leases; and (c) whether previously capitalized IDC for existing leases would qualify for capitalization under the standard. At this time, the Company does not plan to elect the hindsight practical expedient which allows entities to use hindsight when determining lease term and impairment of right-of-use assets. In preparing for adoption, the Company has formed a cross-functional working group comprised of individuals from various disciplines, including procurement, real estate and finance, to evaluate the amended guidance and its potential impact on its Consolidated Financial Statements. To date, the Company has identified several areas that are within the scope of the guidance, including (i) its contracts with respect to leased real estate and office equipment and (ii) lease agreements entered into with customers of the Company’s equipment financing businesses. The Company continues to evaluate various service contracts in order to determine whether embedded leases exist that are deemed to be in scope. At this time, the most significant impact is believed to relate to real estate (primarily branch locations) and office equipment subject to non-cancelable operating lease agreements entered into by the Company as lessee. The amount of the right-of-use assets and corresponding lease liabilities recorded upon adoption will be based, primarily, on the present value of unpaid future minimum lease payments, the amount of which is dependent upon the population of leases in effect at the date of adoption, as well as assumptions with respect to renewals and/or extensions and the interest rate used to discount the future lease obligations. As of December 31, 2017, the Company reported approximately $353 million in future minimum lease payments due under such agreements. While these leases represent a majority of the leases within the scope of the standard, the lease portfolio is subject to change as a result of the execution of new leases and/or termination of existing leases prior to the effective date. Further, the Company will continue to review contracts up through the effective date and may identify additional leases within the scope of the standard. The Federal banking agencies have indicated that to the extent a right-of-use asset arises due to a lease of a tangible asset (e.g. building or equipment), the asset should be: (i) treated as a tangible asset not subject to deduction from regulatory capital; (ii) risk-weighted at 100%; and (iii) included in total assets for leverage capital purposes. As it relates to lease agreements entered into with equipment financing customers, and for which the Company acts as lessor, the impact is most likely to be related to the definition of eligible IDC under the new guidance. Specifically, the standard maintains a narrower definition of IDC which may result in the Company recognizing immediately (rather than deferring) certain lease origination-related expenses. Such expenses would be offset by the recognition of a higher yield on the underlying leases over their contractual term. Premium Amortization – Purchased Callable Debt Securities In April 2017, the FASB amended its standards to shorten the amortization period for certain callable debt securities held at a premium, requiring such premiums to be amortized to the earliest call date unless applicable guidance related to certain pools of securities is applied to consider estimated prepayments. Under prior guidance, entities were generally required to amortize premiums on individual, non-pooled Derivatives and Hedging In August 2017, the FASB amended its standards with respect to the accounting for derivatives and hedging, simplifying existing guidance in order to enable companies to more accurately portray the economic effects of risk management activities in the financial statements and enhancing the transparency and understandability of hedge results through improved disclosures. This new guidance is effective for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018 (January 1, 2019 for People’s United) with early adoption permitted. Given the limited number of derivatives currently designated as hedging instruments, the adoption of this amendment is not expected to have a significant impact on the Company’s Consolidated Financial Statements. Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In February 2018, as a result of the enactment of the Tax Cuts and Jobs Act (the “Act”), the FASB issued new accounting guidance providing entities with the option to reclassify, from accumulated other comprehensive income to retained earnings, certain “stranded tax effects” resulting from application of the Act. An entity that elects to do so must provide the following disclosures in the period of adoption: (i) that an election was made to reclassify the income tax effects of the Act from accumulated other comprehensive income to retained earnings and (ii) a description of other income tax effects related to the application of the Act that were reclassified from accumulated other comprehensive income to retained earnings, if any (e.g. income tax effects other than the effect of the change in the U.S. federal corporate income tax rate on gross deferred tax amounts and related valuation allowances). Regardless of whether an entity elects to adopt the guidance or not it is required to disclose its accounting policy for releasing income tax effects from accumulated other comprehensive income (e.g. the portfolio approach or the security-by-security The guidance is effective for all organizations for fiscal years beginning after December 15, 2018 (January 1, 2019 for People’s United), including interim periods within those fiscal years, and early adoption is permitted. Entities electing to apply the guidance should do so (i) as of the beginning of the period of adoption or (ii) retrospectively to each period in which the effect of the change in the U.S. federal corporate income tax rate is recognized. The Company elected to early adopt this amendment effective January 1, 2018 (for the reporting period ending on March 31, 2018). Upon adoption, $37.9 million, representing the income tax effects of the Act as well as the indirect impacts from the decreased federal tax effect on future state tax benefits, was reclassified from AOCL to retained earnings. The reclassification adjustment, which related to: (i) the net actuarial loss on defined benefit pension and postretirement plans; (ii) the net unrealized loss on debt securities available-for-sale held-to-maturity; Standards effective in 2020 Financial Instruments – Credit Losses In June 2016, the FASB amended its standards with respect to certain aspects of measurement, recognition and disclosure of credit losses on loans and other financial instruments, including available-for-sale In preparing for adoption, the Company has established a cross-functional working group comprised of individuals from various disciplines, including credit, risk management, information technology and finance. That working group, which is subject to the Company’s established corporate governance and oversight structure, is responsible for: (i) identifying key interpretative issues; (ii) determining the appropriate level of portfolio segmentation; (iii) reviewing historical data so as to identify potential data and resource gaps; and (iv) evaluating existing credit loss forecasting models and processes in order to determine what modifications may be required. To date, implementation efforts have focused on: (i) establishing the Company’s portfolio segmentation; (ii) fulfilling the data requirements of the standard according to the established portfolio segmentation; and (iii) purchasing and installing a third-party vendor solution that will aid in the application of the standard. Currently, the working group, in conjunction with an advisory consultant, is devoting considerable time to model selection and development, which includes an assessment of how existing credit models used to comply with other regulatory requirements may be leveraged. In addition, the working group is actively monitoring interpretative guidance issued by standard setters while also considering the relevant internal controls and processes necessary to support the requirements of the new standard. As a result of the required change in approach toward determining estimated credit losses from the current “incurred loss” model to one based on estimated cash flows over a loan’s contractual life, adjusted for prepayments (a “life of loan” model), the Company expects the new guidance will result in an increase in the allowance for loan losses, particularly for longer duration portfolios. The Company also expects the new guidance may result in an allowance for debt securities. In both cases, the extent of the change is indeterminable at this time as it will be dependent upon portfolio composition and credit quality at the adoption date, as well as economic conditions and forecasts at that time. To date, no formal guidance has been issued by either the Company’s or the Bank’s primary regulators with respect to how the impact of the amended standard is to be treated for regulatory capital purposes. However, in April 2018, the federal banking agencies issued a proposal for public comment that would serve to revise the regulatory capital rules to, among other things, provide banks an option to phase-in, Simplifying the Test for Goodwill Impairment In January 2017, the FASB amended its standards with respect to goodwill, simplifying how an entity is required to conduct the impairment assessment by eliminating Step 2, which requires a hypothetical purchase price allocation, from the goodwill impairment test. Instead, goodwill impairment will now be measured as the amount by which a reporting unit’s carrying amount exceeds its fair value, not to exceed the carrying amount of goodwill. An entity will still have the option to perform a qualitative assessment to determine if a quantitative impairment test is necessary. For public business entities, this new guidance is effective in fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019 (January 1, 2020 for People’s United) and is to be applied prospectively. Early adoption is permitted for any impairment tests performed after January 1, 2017. This amendment, which the Company elected to early adopt effective January 1, 2018, is not expected to have a significant impact on the Company’s Consolidated Financial Statements. Disclosure Requirements – Fair Value Measurement In August 2018, the FASB issued targeted amendments that serve to eliminate, add and modify certain disclosure requirements for fair value measurements. Among the changes, entities will no longer be required to disclose the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, but will be required to disclose the range and weighted average used to develop significant unobservable inputs for Level 3 fair value measurements. These amendments are effective for interim and annual reporting periods beginning after December 15, 2019 (January 1, 2020 for People’s United) and early adoption is permitted. Entities may also elect to early adopt the eliminated and/or modified disclosure requirements and delay adoption of the new disclosure requirements until their effective date. As the amendments simply serve to revise disclosure requirements, they are not expected to have a significant impact on the Company’s Consolidated Financial Statements. Standards effective in 2021 Disclosure Requirements – Defined Benefit Plans In August 2018, the FASB issued targeted amendments that serve to make minor changes to the disclosure requirements for employers that sponsor defined benefit pension and/or other postretirement benefit plans. More specifically, the amendments (i) remove disclosures that are no longer considered cost beneficial, (ii) clarify the specific requirements of selected disclosures and (iii) add disclosure requirements identified as relevant. These amendments are effective for fiscal years ending after December 15, 2020 (January 1, 2021 for People’s United) and early adoption is permitted. As the amendments simply serve to revise disclosure requirements, they are not expected to have a significant impact on the Company’s Consolidated Financial Statements.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2018
Subsequent Events [Abstract]
Subsequent Events	NOTE 14. SUBSEQUENT EVENTS Acquisition of First Connecticut Bancorp, Inc. Effective October 1, 2018, People’s United completed its acquisition of First Connecticut Bancorp, Inc. (“First Connecticut”) based in Farmington, Connecticut. Accordingly, People’s United’s consolidated financial statements as of and for the periods ended September 30, 2018 do not include amounts for First Connecticut. At the acquisition date, First Connecticut had total loans of $2.97 billion, total deposits of $2.40 billion and operated 25 branch locations throughout central Connecticut and western Massachusetts. The fair value of the consideration transferred in the First Connecticut acquisition totaled $486.4 million and consisted of 28.4 million shares of People’s United common stock. Merger-related expenses totaling $1.5 million relating to this transaction were recorded in the nine months ended September 30, 2018.

Employee Benefit Plans (Policie

Employee Benefit Plans (Policies)	9 Months Ended
	Sep. 30, 2018
People's United Employee Pension and Other Postretirement Plans	People’s United Employee Pension and Other Postretirement Plans People’s United maintains a qualified noncontributory defined benefit pension plan (the “People’s Qualified Plan”) that covers substantially all full-time and part-time employees who (i) meet certain age and length of service requirements and (ii) were employed by the Bank prior to August 14, 2006. Benefits are based upon the employee’s years of credited service and either the average compensation for the last five years or the average compensation for the five consecutive years of the last ten years that produce the highest average. New employees of the Bank starting on or after August 14, 2006 are not eligible to participate in the People’s Qualified Plan. Instead, the Bank makes contributions on behalf of these employees to a qualified defined contribution plan in an annual amount equal to 3% of the employee’s eligible compensation. Employee participation in this plan is restricted to employees who (i) are at least 18 years of age and (ii) worked at least 1,000 hours in a year. Both full-time and part-time employees are eligible to participate as long as they meet these requirements. In July 2011, the Bank amended the People’s Qualified Plan to “freeze”, effective December 31, 2011, the accrual of pension benefits for People’s Qualified Plan participants. As such, participants will not earn any additional benefits after that date. Instead, effective January 1, 2012, the Bank began making contributions on behalf of these participants to a qualified defined contribution plan in an annual amount equal to 3% of the employee’s eligible compensation. In addition to the People’s Qualified Plan, People’s United continues to maintain a qualified defined benefit pension plan that covers former Chittenden employees who meet certain eligibility requirements (the “Chittenden Qualified Plan”). Effective December 31, 2005, accrued benefits were frozen based on participants’ then-current service and pay levels. Interest continues to be credited on undistributed balances at a crediting rate specified by the Chittenden Qualified Plan. During April 2010, participants who were in payment status as of April 1, 2010, or whose accrued benefit as of that date was scheduled to be paid in the form of an annuity commencing May 1, 2010 based upon elections made by April 15, 2010, were transferred into the People’s Qualified Plan. People’s United also continues to maintain a qualified defined benefit pension plan that covers former Suffolk Bancorp (“Suffolk”) employees who meet certain eligibility requirements (the “Suffolk Qualified Plan”). Effective December 31, 2012, accrued benefits were frozen based on participants’ then-current service and pay levels. Interest continues to be credited on undistributed balances at a crediting rate specified by the Suffolk Qualified Plan. People’s United’s funding policy is to contribute the amounts required by applicable regulations, although additional amounts may be contributed from time to time. In February 2018, People’s United made voluntary employer contributions of $40 million to the People’s Qualified Plan and $10 million to the Chittenden Qualified Plan (none to the Suffolk Qualified Plan) in response to tax reform. People’s United also maintains (i) unfunded, nonqualified supplemental plans to provide retirement benefits to certain senior officers (the “Supplemental Plans”) and (ii) an unfunded plan that provides retirees with optional medical, dental and life insurance benefits (the “Other Postretirement Plan”). People’s United accrues the cost of these postretirement benefits over the employees’ years of service to the date of their eligibility for such benefit.
Employee Stock Ownership Plan	Employee Stock Ownership Plan In April 2007, People’s United established an ESOP. At that time, People’s United loaned the ESOP $216.8 million to purchase 10,453,575 shares of People’s United common stock in the open market. In order for the ESOP to repay the loan, People’s United expects to make annual cash contributions of approximately $18.8 million until 2036. Such cash contributions may be reduced by the cash dividends paid on unallocated ESOP shares, which, for the nine months ended September 30, 2018, totaled $3.5 million. At September 30, 2018, the loan balance totaled $180.8 million. Employee participation in this plan is restricted to those employees who (i) are at least 18 years of age and (ii) worked at least 1,000 hours within 12 months of their hire date or any plan year (January 1 to December 31) after their date of hire. Employees meeting the aforementioned eligibility criteria during the plan year must continue to be employed as of the last day of the plan year in order to receive an allocation of shares for that plan year. Shares of People’s United common stock are held by the ESOP and allocated to eligible participants annually based upon a percentage of each participant’s eligible compensation. Since the ESOP was established, a total of 4,094,316 shares of People’s United common stock have been allocated or committed to be released to participants’ accounts. At September 30, 2018, 6,359,259 shares of People’s United common stock, with a fair value of $108.9 million at that date, have not been allocated or committed to be released. Compensation expense related to the ESOP is recognized at an amount equal to the number of common shares committed to be released by the ESOP for allocation to participants’ accounts multiplied by the average fair value of People’s United’s common stock during the reporting period. The difference between the fair value of the shares of People’s United’s common stock committed to be released and the cost of those common shares is recorded as a credit to additional paid-in paid-in
Fair Value, Measurements, Recurring [Member]
Fair Value Measurements	Recurring Fair Value Measurements Trading Debt Securities, Equity Securities and Debt Securities Available-For-Sale When available, People’s United uses quoted market prices for identical securities received from an independent, nationally-recognized, third-party pricing service (as discussed further below) to determine the fair value of investment securities such as U.S. Treasury and agency securities and equity securities that are included in Level 1. When quoted market prices for identical securities are unavailable, People’s United uses prices provided by the independent pricing service based on recent trading activity and other observable information including, but not limited to, market interest rate curves, referenced credit spreads and estimated prepayment rates where applicable. These investments include certain U.S. and government agency debt securities, corporate and municipal debt securities, and GSE mortgage-backed securities, all of which are included in Level 2. The Company’s available-for-sale available-for-sale 10- 15-year
Fair Value, Measurements, Nonrecurring [Member]
Fair Value Measurements	Non-Recurring Loans Held-for-Sale Residential mortgage loans held-for-sale non-recurring Impaired Loans Loan impairment is deemed to exist when full repayment of principal and interest according to the contractual terms of the loan is no longer probable. Impaired loans are reported based on one of three measures: (i) the present value of expected future cash flows discounted at the loan’s original effective interest rate; (ii) the loan’s observable market price; or (iii) the fair value of the collateral (less estimated cost to sell) if the loan is collateral dependent. Accordingly, certain impaired loans may be subject to measurement at fair value on a non-recurring People’s United has estimated the fair values of these assets using Level 3 inputs, such as discounted cash flows based on inputs that are largely unobservable and, instead, reflect management’s own estimates of the assumptions a market participant would use in pricing such loans and/or the fair value of collateral based on independent third-party appraisals for collateral-dependent loans. Such appraisals are based on the market and/or income approach to value and are subject to a discount (to reflect estimated cost to sell) that generally approximates 10%. REO and Repossessed Assets REO and repossessed assets are recorded at the lower of cost or fair value, less estimated selling costs, and are therefore measured at fair value on a non-recurring held-for-sale

Cash and Cash Equivalents and_2

Cash and Cash Equivalents and Securities (Tables)	9 Months Ended
	Sep. 30, 2018
Cash and Cash Equivalents [Abstract]
Available-for-Sale and Held-to-Maturity Debt Securities Gains (Losses)	The amortized cost, gross unrealized gains and losses, and fair value of People’s United’s debt securities available-for-sale held-to-maturity As of September 30, 2018 (in millions) Amortized Gross Gross Fair Debt securities available-for-sale: U.S. Treasury and agency $ 683.5 $ — $ (31.4 ) $ 652.1 GSE (1) mortgage-backed securities 2,742.5 1.4 (83.9 ) 2,660.0 Total debt securities available-for-sale $ 3,426.0 $ 1.4 $ (115.3 ) $ 3,312.1 Debt securities held-to-maturity: State and municipal $ 2,268.2 $ 19.7 $ (37.4 ) $ 2,250.5 GSE mortgage-backed securities 1,407.3 — (57.7 ) 1,349.6 Corporate 65.9 — (0.7 ) 65.2 Other 1.5 — — 1.5 Total debt securities held-to-maturity $ 3,742.9 $ 19.7 $ (95.8 ) $ 3,666.8 (1) Government sponsored enterprise As of December 31, 2017 (in millions) Amortized Gross Gross Fair Debt securities available-for-sale: U.S. Treasury and agency $ 687.1 $ — $ (18.3 ) $ 668.8 GSE mortgage-backed securities 2,477.8 7.4 (28.7 ) 2,456.5 Total debt securities available-for-sale $ 3,164.9 $ 7.4 $ (47.0 ) $ 3,125.3 Debt securities held-to-maturity: State and municipal $ 2,060.4 $ 64.5 $ (4.6 ) $ 2,120.3 GSE mortgage-backed securities 1,474.9 0.3 (15.4 ) 1,459.8 Corporate 51.3 0.8 — 52.1 Other 1.5 — — 1.5 Total debt securities held-to-maturity $ 3,588.1 $ 65.6 $ (20.0 ) $ 3,633.7
Summary of Amortized Cost and Fair Value of Debt Securities Based on Remaining Period to Contractual Maturity	The following table is a summary of the amortized cost and fair value of debt securities as of September 30, 2018, based on remaining period to contractual maturity. Information for GSE mortgage-backed securities is based on the final contractual maturity dates without considering repayments and prepayments. Available-for-Sale Held-to-Maturity (in millions) Amortized Fair Amortized Fair U.S. Treasury and agency: Within 1 year $ 6.0 $ 6.0 $ — $ — After 1 but within 5 years 677.5 646.1 — — Total 683.5 652.1 — — GSE mortgage-backed securities: Within 1 year — — 4.3 4.3 After 1 but within 5 years 53.6 52.9 234.7 226.4 After 5 but within 10 years 1,014.6 1,000.8 800.4 764.9 After 10 years 1,674.3 1,606.3 367.9 354.0 Total 2,742.5 2,660.0 1,407.3 1,349.6 State and municipal: Within 1 year — — 23.5 23.5 After 1 but within 5 years — — 180.3 183.2 After 5 but within 10 years — — 334.4 340.1 After 10 years — — 1,730.0 1,703.7 Total — — 2,268.2 2,250.5 Corporate: After 1 but within 5 years — — 5.0 5.0 After 5 but within 10 years — — 60.9 60.2 Total — — 65.9 65.2 Other: After 1 but within 5 years — — 1.5 1.5 Total — — 1.5 1.5 Total: Within 1 year 6.0 6.0 27.8 27.8 After 1 but within 5 years 731.1 699.0 421.5 416.1 After 5 but within 10 years 1,014.6 1,000.8 1,195.7 1,165.2 After 10 years 1,674.3 1,606.3 2,097.9 2,057.7 Total $ 3,426.0 $ 3,312.1 $ 3,742.9 $ 3,666.8
Continuous Unrealized Loss Position on Available-for-Sale and Held-to-Maturities Securities	The following tables summarize those debt securities with unrealized losses, segregated by the length of time the securities have been in a continuous unrealized loss position, at the respective dates. Certain unrealized losses totaled less than $0.1 million. Continuous Unrealized Loss Position Less Than 12 Months 12 Months Or Longer Total As of September 30, 2018 (in millions) Fair Unrealized Fair Unrealized Fair Unrealized Debt securities available-for-sale: GSE mortgage-backed securities $ 893.6 $ (14.2 ) $ 1,604.7 $ (69.7 ) $ 2,498.3 $ (83.9 ) U.S. Treasury and agency 152.5 (1.8 ) 494.6 (29.6 ) 647.1 (31.4 ) Debt securities held-to-maturity: GSE mortgage-backed securities 330.7 (11.9 ) 1,018.9 (45.8 ) 1,349.6 (57.7 ) State and municipal 1,048.1 (21.6 ) 265.4 (15.8 ) 1,313.5 (37.4 ) Corporate 51.4 (0.7 ) — — 51.4 (0.7 ) Total $ 2,476.3 $ (50.2 ) $ 3,383.6 $ (160.9 ) $ 5,859.9 $ (211.1 ) Continuous Unrealized Loss Position Less Than 12 Months 12 Months Or Longer Total As of December 31, 2017 (in millions) Fair Unrealized Fair Unrealized Fair Unrealized Debt securities available-for-sale: GSE mortgage-backed securities $ 1,013.5 $ (8.7 ) $ 1,114.8 $ (20.0 ) $ 2,128.3 $ (28.7 ) U.S. Treasury and agency 156.0 — 507.7 (18.3 ) 663.7 (18.3 ) Debt securities held-to-maturity: GSE mortgage-backed securities 1,289.3 (14.7 ) 45.0 (0.7 ) 1,334.3 (15.4 ) State and municipal 106.2 (0.5 ) 224.9 (4.1 ) 331.1 (4.6 ) Total $ 2,565.0 $ (23.9 ) $ 1,892.4 $ (43.1 ) $ 4,457.4 $ (67.0 )

Loans (Tables)

Loans (Tables)	9 Months Ended
	Sep. 30, 2018
Receivables [Abstract]
Summary of Loans by Loan Portfolio Segment and Class	The following table summarizes People’s United’s loans by loan portfolio segment and class: September 30, 2018 December 31, 2017 (in millions) Originated Acquired Total Originated Acquired Total Commercial: Commercial real estate $ 9,750.9 $ 844.6 $ 10,595.5 $ 10,126.6 $ 942.1 $ 11,068.7 Commercial and industrial 8,078.9 489.7 8,568.6 8,129.9 601.2 8,731.1 Equipment financing 3,734.9 474.4 4,209.3 3,308.5 596.9 3,905.4 Total Commercial Portfolio 21,564.7 1,808.7 23,373.4 21,565.0 2,140.2 23,705.2 Retail: Residential mortgage: Adjustable-rate 5,810.8 126.7 5,937.5 5,782.6 144.0 5,926.6 Fixed-rate 867.5 106.9 974.4 758.0 121.1 879.1 Total residential mortgage 6,678.3 233.6 6,911.9 6,540.6 265.1 6,805.7 Home equity and other consumer: Home equity 1,823.6 43.6 1,867.2 1,960.0 55.2 2,015.2 Other consumer 44.2 2.6 46.8 45.6 3.6 49.2 Total home equity and other consumer 1,867.8 46.2 1,914.0 2,005.6 58.8 2,064.4 Total Retail Portfolio 8,546.1 279.8 8,825.9 8,546.2 323.9 8,870.1 Total loans $ 30,110.8 $ 2,088.5 $ 32,199.3 $ 30,111.2 $ 2,464.1 $ 32,575.3
Summary, by Loan Portfolio Segment, of Activity in Allowance for Loan Losses	The following tables present a summary, by loan portfolio segment, of activity in the allowance for loan losses for the three and nine months ended September 30, 2018 and 2017. With respect to the originated portfolio, an allocation of a portion of the allowance to one segment does not preclude its availability to absorb losses in another segment. Three months ended Commercial Retail September 30, 2018 (in millions) Originated Acquired Total Originated Acquired Total Total Balance at beginning of period $ 202.0 $ 3.8 $ 205.8 $ 30.8 $ 0.2 $ 31.0 $ 236.8 Charge-offs (5.7 ) (2.0 ) (7.7 ) (0.7 ) — (0.7 ) (8.4 ) Recoveries 0.6 0.4 1.0 0.4 — 0.4 1.4 Net loan charge-offs (5.1 ) (1.6 ) (6.7 ) (0.3 ) — (0.3 ) (7.0 ) Provision for loan losses 6.3 1.7 8.0 0.2 — 0.2 8.2 Balance at end of period $ 203.2 $ 3.9 $ 207.1 $ 30.7 $ 0.2 $ 30.9 $ 238.0 Nine months ended Commercial Retail September 30, 2018 (in millions) Originated Acquired Total Originated Acquired Total Total Balance at beginning of period $ 201.1 $ 3.4 $ 204.5 $ 29.7 $ 0.2 $ 29.9 $ 234.4 Charge-offs (12.9 ) (6.3 ) (19.2 ) (2.6 ) — (2.6 ) (21.8 ) Recoveries 2.6 1.0 3.6 1.7 — 1.7 5.3 Net loan charge-offs (10.3 ) (5.3 ) (15.6 ) (0.9 ) — (0.9 ) (16.5 ) Provision for loan losses 12.4 5.8 18.2 1.9 — 1.9 20.1 Balance at end of period $ 203.2 $ 3.9 $ 207.1 $ 30.7 $ 0.2 $ 30.9 $ 238.0 Three months ended Commercial Retail September 30, 2017 (in millions) Originated Acquired Total Originated Acquired Total Total Balance at beginning of period $ 198.3 $ 3.6 $ 201.9 $ 29.6 $ 0.1 $ 29.7 $ 231.6 Charge-offs (4.0 ) (1.0 ) (5.0 ) (1.8 ) — (1.8 ) (6.8 ) Recoveries 0.9 0.1 1.0 0.6 — 0.6 1.6 Net loan charge-offs (3.1 ) (0.9 ) (4.0 ) (1.2 ) — (1.2 ) (5.2 ) Provision for loan losses 4.3 1.3 5.6 1.3 0.1 1.4 7.0 Balance at end of period $ 199.5 $ 4.0 $ 203.5 $ 29.7 $ 0.2 $ 29.9 $ 233.4 Nine months ended Commercial Retail September 30, 2017 (in millions) Originated Acquired Total Originated Acquired Total Total Balance at beginning of period $ 198.8 $ 6.1 $ 204.9 $ 24.2 $ 0.2 $ 24.4 $ 229.3 Charge-offs (12.0 ) (2.9 ) (14.9 ) (5.1 ) — (5.1 ) (20.0 ) Recoveries 3.8 0.1 3.9 1.7 — 1.7 5.6 Net loan charge-offs (8.2 ) (2.8 ) (11.0 ) (3.4 ) — (3.4 ) (14.4 ) Provision for loan losses 8.9 0.7 9.6 8.9 8.9 18.5 Balance at end of period $ 199.5 $ 4.0 $ 203.5 $ 29.7 $ 0.2 $ 29.9 $ 233.4
Summary of Allowance for Loan Losses by Loan Portfolio Segment and Impairment Methodology	The following tables summarize, by loan portfolio segment and impairment methodology, the allowance for loan losses and related portfolio balances: As of September 30, 2018 Commercial Retail Total (in millions) Portfolio Allowance Portfolio Allowance Portfolio Allowance Originated loans: Collectively evaluated for impairment $ 21,441.6 $ 195.2 $ 8,458.9 $ 28.4 $ 29,900.5 $ 223.6 Individually evaluated for impairment 123.1 8.0 87.2 2.3 210.3 10.3 Acquired loans: PCI (1) 284.3 2.2 110.3 0.1 394.6 2.3 Purchased performing: Collectively evaluated for impairment 1,521.2 1.7 167.9 — 1,689.1 1.7 Individually evaluated for impairment 3.2 — 1.6 0.1 4.8 0.1 Total $ 23,373.4 $ 207.1 $ 8,825.9 $ 30.9 $ 32,199.3 $ 238.0 As of December 31, 2017 Commercial Retail Total (in millions) Portfolio Allowance Portfolio Allowance Portfolio Allowance Originated loans: Collectively evaluated for impairment $ 21,423.8 $ 196.5 $ 8,454.1 $ 27.3 $ 29,877.9 $ 223.8 Individually evaluated for impairment 141.2 4.6 92.1 2.4 233.3 7.0 Acquired loans: PCI (1) 370.4 2.8 128.1 0.2 498.5 3.0 Purchased performing: Collectively evaluated for impairment 1,769.8 0.6 193.9 — 1,963.7 0.6 Individually evaluated for impairment — — 1.9 — 1.9 — Total $ 23,705.2 $ 204.5 $ 8,870.1 $ 29.9 $ 32,575.3 $ 234.4 (1) Purchased credit impaired (“PCI”) loans are evaluated for impairment on a pool basis.
Summarized Recorded Investments, by Class of Loan, in Originated Non-Performing Loans	The recorded investments, by class of loan, in originated non-performing September 30, December 31, (in millions) 2018 2017 Commercial: Commercial real estate $ 17.2 $ 23.7 Commercial and industrial 44.9 32.6 Equipment financing 49.3 44.3 Total (1) 111.4 100.6 Retail: Residential mortgage 32.0 32.7 Home equity 14.6 15.4 Other consumer 0.1 — Total (2) 46.7 48.1 Total $ 158.1 $ 148.7 (1) Reported net of government guarantees totaling $2.5 million and $3.1 million at September 30, 2018 and December 31, 2017, respectively. These government guarantees relate, almost entirely, to guarantees provided by the Small Business Administration as well as selected other Federal agencies and represent the carrying value of the loans that are covered by such guarantees, the extent of which (i.e. full or partial) varies by loan. At September 30, 2018, the principal loan classes to which these government guarantees relate are commercial and industrial loans (93%) and commercial real estate loans (7%). (2) Includes $17.6 million and $15.2 million of loans in the process of foreclosure at September 30, 2018 and December 31, 2017, respectively.
Summary of Recorded Investments in TDRs by Class of Loan	The following tables summarize, by class of loan, the recorded investments in loans modified as TDRs during the three and nine months ended September 30, 2018 and 2017. For purposes of this disclosure, recorded investments represent amounts immediately prior to and subsequent to the restructuring. Three Months Ended September 30, 2018 Pre-Modification Post-Modification Outstanding Outstanding Number Recorded Recorded (dollars in millions) of Contracts Investment Investment Commercial: Commercial real estate (1) 4 $ 20.7 $ 20.7 Commercial and industrial (2) 10 10.3 10.3 Equipment financing (3) 5 10.3 10.3 Total 19 41.3 41.3 Retail: Residential mortgage (4) 4 1.4 1.4 Home equity (5) 19 2.2 2.2 Other consumer — — — Total 23 3.6 3.6 Total 42 $ 44.9 $ 44.9 (1) Represents the following concessions: extension of term (3 contracts; recorded investment of $20.2 million); or reduced payment and/or payment deferral (1 contract; recorded investment of $0.5 million). (2) Represents the following concessions: extension of term (6 contracts; recorded investment of $3.6 million); reduced payment and/or payment deferral (2 contracts; recorded investment of $6.4 million); or a combination of concessions (2 contracts; recorded investment of $0.3 million). (3) Represents the following concessions: extension of term (2 contracts; recorded investment of $3.3 million); reduced payment and/or payment deferral (2 contracts; recorded investment of $5.9 million); or a combination of concessions (1 contract; recorded investment of $1.1 million). (4) Represents the following concessions: loans restructured through bankruptcy (2 contracts; recorded investment of $0.6 million); reduced payment and/or payment deferral (1 contract; recorded investment of $0.7 million); or a combination of concessions (1 contract; recorded investment of $0.1 million). (5) Represents the following concessions: loans restructured through bankruptcy (11 contracts; recorded investment of $1.1 million); reduced payment and/or payment deferral (6 contracts; recorded investment of $1.0 million); or a combination of concessions (2 contracts; recorded investment of $0.1 million). Nine Months Ended September 30, 2018 (dollars in millions) Number Pre-Modification Post-Modification Commercial: Commercial real estate (1) 9 $ 24.3 $ 24.3 Commercial and industrial (2) 34 55.2 55.2 Equipment financing (3) 16 20.4 20.4 Total 59 99.9 99.9 Retail: Residential mortgage (4) 16 4.9 4.9 Home equity (5) 56 4.9 4.9 Other consumer — — — Total 72 9.8 9.8 Total 131 $ 109.7 $ 109.7 (1) Represents the following concessions: extension of term (8 contracts; recorded investment of $23.8 million); or reduced payment and/or payment deferral (1 contract; recorded investment of $0.5 million). (2) Represents the following concessions: extension of term (21 contracts; recorded investment of $30.7 million); reduced payment and/or payment deferral (10 contracts; recorded investment of $23.8 million); or a combination of concessions (3 contracts; recorded investment of $0.7 million). (3) Represents the following concessions: extension of term (2 contracts; recorded investment of $3.3 million); reduced payment and/or payment deferral (8 contracts; recorded investment of $12.9 million); or a combination of concessions (6 contracts; recorded investment of $4.2 million). (4) Represents the following concessions: loans restructured through bankruptcy (5 contracts; recorded investment of $0.9 million); reduced payment and/or payment deferral (6 contracts; recorded investment of $2.4 million); or a combination of concessions (5 contracts; recorded investment of $1.6 million). (5) Represents the following concessions: loans restructured through bankruptcy (37 contracts; recorded investment of $2.8 million); reduced payment and/or payment deferral (10 contracts; recorded investment of $1.3 million); or a combination of concessions (9 contracts; recorded investment of $0.8 million). Three Months Ended September 30, 2017 (dollars in millions) Number Pre-Modification Post-Modification Commercial: Commercial real estate (1) 1 $ 0.1 $ 0.1 Commercial and industrial (2) 9 11.6 11.6 Equipment financing (3) 13 8.1 8.1 Total 23 19.8 19.8 Retail: Residential mortgage (4) 17 3.8 3.8 Home equity (5) 19 1.5 1.5 Other consumer — — — Total 36 5.3 5.3 Total 59 $ 25.1 $ 25.1 (1) Represents the following concession: extension of term (1 contract; recorded investment of $0.1 million). (2) Represents the following concessions: extension of term (9 contracts; recorded investment of $11.6 million). (3) Represents the following concessions: reduced payment and/or payment deferral (12 contracts; recorded investment of $8.0 million); or a combination of concessions (1 contract; recorded investment of $0.1 million). (4) Represents the following concessions: loans restructured through bankruptcy (9 contracts; recorded investment of $1.7 million); reduced payment and/or payment deferral (5 contract; recorded investment of $1.7 million); or a combination of concessions (3 contracts; recorded investment of $0.4 million). (5) Represents the following concessions: loans restructured through bankruptcy (10 contracts; recorded investment of $1.0 million); reduced payment and/or payment deferral (4 contracts; recorded investment of $0.3 million); or a combination of concessions (5 contracts; recorded investment of $0.2 million). Nine Months Ended September 30, 2017 (dollars in millions) Number Pre-Modification Post-Modification Commercial: Commercial real estate (1) 8 $ 5.3 $ 5.3 Commercial and industrial (2) 28 39.6 39.6 Equipment financing (3) 54 25.1 25.1 Total 90 70.0 70.0 Retail: Residential mortgage (4) 45 10.8 10.8 Home equity (5) 65 4.5 4.5 Other consumer — — — Total 110 15.3 15.3 Total 200 $ 85.3 $ 85.3 (1) Represents the following concessions: extension of term (5 contracts; recorded investment of $1.4 million); reduced payment and/or payment deferral (2 contracts; recorded investment of $2.2 million); or a temporary rate reduction (1 contract; recorded investment of $1.7 million). (2) Represents the following concessions: extension of term (25 contracts; recorded investment of $29.9 million); reduced payment and/or payment deferral (2 contracts; recorded investment of $9.4 million); or a combination of concessions (1 contract; recorded investment of $0.3 million). (3) Represents the following concessions: reduced payment and/or payment deferral (28 contracts; recorded investment of $17.4 million); or a combination of concessions (26 contracts; recorded investment of $7.7 million). (4) Represents the following concessions: loans restructured through bankruptcy (28 contracts; recorded investment of $4.2 million); reduced payment and/or payment deferral (10 contracts; recorded investment of $3.4 million); or a combination of concessions (7 contracts; recorded investment of $3.2 million). (5) Represents the following concessions: loans restructured through bankruptcy (41 contracts; recorded investment of $2.6 million); reduced payment and/or payment deferral (10 contracts; recorded investment of $0.8 million); or a combination of concessions (14 contracts; recorded investment of $1.1 million).
Summary of Recorded Investments in TDRs by Class of Loan, Subsequently Defaulted	The following is a summary, by class of loan, of information related to TDRs completed within the previous 12 months that subsequently defaulted during the three and nine months ended September 30, 2018 and 2017. For purposes of this disclosure, the previous 12 months is measured from October 1 of the respective prior year and a default represents a previously-modified loan that became past due 30 days or more during the three or nine months ended September 30, 2018 or 2017. Three Months Ended September 30, 2018 2017 (dollars in millions) Number Recorded Number Recorded Commercial: Commercial real estate — $ — — $ — Commercial and industrial 4 5.0 1 0.1 Equipment financing 1 0.2 8 4.2 Total 5 5.2 9 4.3 Retail: Residential mortgage 5 0.9 2 0.9 Home equity 8 0.6 5 0.8 Other consumer — — — — Total 13 1.5 7 1.7 Total 18 $ 6.7 16 $ 6.0 Nine Months Ended September 30, 2018 2017 (dollars in millions) Number Recorded Number Recorded Commercial: Commercial real estate 2 $ 0.8 — $ — Commercial and industrial 13 8.5 4 1.5 Equipment financing 7 6.8 15 6.8 Total 22 16.1 19 8.3 Retail: Residential mortgage 7 1.3 9 2.7 Home equity 9 0.6 11 1.3 Other consumer — — — — Total 16 1.9 20 4.0 Total 38 $ 18.0 39 $ 12.3
Summary of Individually-Evaluated Impaired Loans by Class of Loan	People’s United’s impaired loans consist of certain loans that have been placed on non-accrual As of September 30, 2018 As of December 31, 2017 (in millions) Unpaid Recorded Related Unpaid Recorded Related Without a related allowance for loan losses: Commercial: Commercial real estate $ 31.2 $ 29.2 $ — $ 37.7 $ 36.3 $ — Commercial and industrial 29.3 27.1 — 27.9 25.5 — Equipment financing 31.3 28.0 — 36.9 32.8 — Retail: Residential mortgage 64.0 56.8 — 67.6 60.8 — Home equity 23.5 19.9 — 24.0 20.2 — Other consumer — — — — — — Total $ 179.3 $ 161.0 $ — $ 194.1 $ 175.6 $ — With a related allowance for loan losses: Commercial: Commercial real estate $ 10.3 $ 7.4 $ 0.8 $ 11.7 $ 9.9 $ 0.9 Commercial and industrial 25.9 22.7 4.2 26.9 26.0 2.6 Equipment financing 12.6 11.9 3.0 11.6 10.7 1.1 Retail: Residential mortgage 10.5 10.5 1.8 11.4 11.4 1.7 Home equity 1.6 1.6 0.6 1.7 1.6 0.7 Other consumer — — — — — — Total $ 60.9 $ 54.1 $ 10.4 $ 63.3 $ 59.6 $ 7.0 Total impaired loans: Commercial: Commercial real estate $ 41.5 $ 36.6 $ 0.8 $ 49.4 $ 46.2 $ 0.9 Commercial and industrial 55.2 49.8 4.2 54.8 51.5 2.6 Equipment financing 43.9 39.9 3.0 48.5 43.5 1.1 Total 140.6 126.3 8.0 152.7 141.2 4.6 Retail: Residential mortgage 74.5 67.3 1.8 79.0 72.2 1.7 Home equity 25.1 21.5 0.6 25.7 21.8 0.7 Other consumer — — — — — — Total 99.6 88.8 2.4 104.7 94.0 2.4 Total $ 240.2 $ 215.1 $ 10.4 $ 257.4 $ 235.2 $ 7.0
Schedule of Impaired Financing Receivable	The following tables summarize, by class of loan, the average recorded investment and interest income recognized on impaired loans for the periods indicated. The average recorded investment amounts are based on month-end Three Months Ended September 30, 2018 2017 (in millions) Average Interest Average Interest Commercial: Commercial real estate $ 36.2 $ 0.4 $ 61.8 $ 0.3 Commercial and industrial 52.4 0.2 59.7 0.7 Equipment financing 36.0 0.1 46.5 0.2 Total 124.6 0.7 168.0 1.2 Retail: Residential mortgage 68.6 0.4 72.4 0.4 Home equity 20.7 0.2 21.8 0.1 Other consumer — — — — Total 89.3 0.6 94.2 0.5 Total $ 213.9 $ 1.3 $ 262.2 $ 1.7 Nine Months Ended September 30, 2018 2017 (in millions) Average Interest Average Interest Commercial: Commercial real estate $ 40.7 $ 0.9 $ 56.9 $ 0.9 Commercial and industrial 50.6 1.3 66.1 1.6 Equipment financing 39.8 0.2 43.0 0.4 Total 131.1 2.4 166.0 2.9 Retail: Residential mortgage 69.5 1.3 71.9 1.3 Home equity 21.0 0.4 21.1 0.3 Other consumer — — — — Total 90.5 1.7 93.0 1.6 Total $ 221.6 $ 4.1 $ 259.0 $ 4.5
Summary of Aging Information by Class of Loan	The following tables summarize, by class of loan, aging information for originated loans: Past Due As of September 30, 2018 (in millions) Current 30-89 90 Days Total Total Commercial: Commercial real estate $ 9,728.2 $ 10.1 $ 12.6 $ 22.7 $ 9,750.9 Commercial and industrial 8,044.5 14.2 20.2 34.4 8,078.9 Equipment financing 3,642.4 75.6 16.9 92.5 3,734.9 Total 21,415.1 99.9 49.7 149.6 21,564.7 Retail: Residential mortgage 6,625.3 31.9 21.1 53.0 6,678.3 Home equity 1,810.6 6.4 6.6 13.0 1,823.6 Other consumer 44.0 0.1 0.1 0.2 44.2 Total 8,479.9 38.4 27.8 66.2 8,546.1 Total originated loans $ 29,895.0 $ 138.3 $ 77.5 $ 215.8 $ 30,110.8 Included in the “Current” and “30-89 non-performing non-accrual Past Due As of December 31, 2017 (in millions) Current 30-89 90 Days Total Total Commercial: Commercial real estate $ 10,102.3 $ 11.0 $ 13.3 $ 24.3 $ 10,126.6 Commercial and industrial 8,099.0 14.9 16.0 30.9 8,129.9 Equipment financing 3,219.7 83.1 5.7 88.8 3,308.5 Total 21,421.0 109.0 35.0 144.0 21,565.0 Retail: Residential mortgage 6,487.3 32.8 20.5 53.3 6,540.6 Home equity 1,945.2 7.4 7.4 14.8 1,960.0 Other consumer 45.3 0.3 — 0.3 45.6 Total 8,477.8 40.5 27.9 68.4 8,546.2 Total originated loans $ 29,898.8 $ 149.5 $ 62.9 $ 212.4 $ 30,111.2
Summary of Credit Quality Indicators by Class of Loan	The following is a summary, by class of loan, of credit quality indicators: As of September 30, 2018 (in millions) Commercial Commercial Equipment Total Commercial: Originated loans: Pass $ 9,500.0 $ 7,644.2 $ 3,352.3 $ 20,496.5 Special mention 134.2 170.1 68.5 372.8 Substandard 115.9 264.2 314.1 694.2 Doubtful 0.8 0.4 — 1.2 Total originated loans 9,750.9 8,078.9 3,734.9 21,564.7 Acquired loans: Pass 779.9 425.8 468.1 1,673.8 Special mention 26.2 9.3 3.2 38.7 Substandard 38.5 54.6 3.1 96.2 Doubtful — — — — Total acquired loans 844.6 489.7 474.4 1,808.7 Total $ 10,595.5 $ 8,568.6 $ 4,209.3 $ 23,373.4 As of September 30, 2018 (in millions) Residential Home Other Total Retail: Originated loans: Low risk $ 3,343.1 $ 862.0 $ 28.2 $ 4,233.3 Moderate risk 2,829.0 589.6 6.6 3,425.2 High risk 506.2 372.0 9.4 887.6 Total originated loans 6,678.3 1,823.6 44.2 8,546.1 Acquired loans: Low risk 136.6 — — 136.6 Moderate risk 52.3 — — 52.3 High risk 44.7 43.6 2.6 90.9 Total acquired loans 233.6 43.6 2.6 279.8 Total $ 6,911.9 $ 1,867.2 $ 46.8 $ 8,825.9 As of December 31, 2017 (in millions) Commercial Commercial Equipment Total Commercial: Originated loans: Pass $ 9,859.3 $ 7,760.7 $ 2,899.9 $ 20,519.9 Special mention 159.4 124.0 91.8 375.2 Substandard 107.0 244.2 316.8 668.0 Doubtful 0.9 1.0 — 1.9 Total originated loans 10,126.6 8,129.9 3,308.5 21,565.0 Acquired loans: Pass 892.0 520.0 596.9 2,008.9 Special mention 14.8 15.2 — 30.0 Substandard 35.3 66.0 — 101.3 Doubtful — — — — Total acquired loans 942.1 601.2 596.9 2,140.2 Total $ 11,068.7 $ 8,731.1 $ 3,905.4 $ 23,705.2 As of December 31, 2017 (in millions) Residential Home Other Total Retail: Originated loans: Low risk $ 3,292.1 $ 925.6 $ 28.2 $ 4,245.9 Moderate risk 2,738.8 640.0 7.1 3,385.9 High risk 509.7 394.4 10.3 914.4 Total originated loans 6,540.6 1,960.0 45.6 8,546.2 Acquired loans: Low risk 148.0 — — 148.0 Moderate risk 65.7 — — 65.7 High risk 51.4 55.2 3.6 110.2 Total acquired loans 265.1 55.2 3.6 323.9 Total $ 6,805.7 $ 2,015.2 $ 49.2 $ 8,870.1
Summarized Activity in Accretable Yield for PCI Loan Portfolio	The following table summarizes activity in the accretable yield for the PCI loan portfolio: Three Months Ended Nine Months Ended September 30, September 30, (in millions) 2018 2017 2018 2017 Balance at beginning of period $ 190.2 $ 233.4 $ 219.7 $ 255.4 Acquisitions — — — 13.1 Accretion (5.6 ) (7.1 ) (17.9 ) (22.5 ) Reclassification from nonaccretable difference for loans with improved cash flows (1) — — — — Other changes in expected cash flows (2) — — (17.2 ) (19.7 ) Balance at end of period $ 184.6 $ 226.3 $ 184.6 $ 226.3 (1) Results in increased interest accretion as a prospective yield adjustment over the remaining life of the corresponding pool of loans. (2) Represents changes in cash flows expected to be collected due to factors other than credit (e.g. changes in prepayment assumptions and/or changes in interest rates on variable rate loans), as well as loan sales, modifications and payoffs.

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	9 Months Ended
	Sep. 30, 2018
Federal Home Loan Banks [Abstract]
Schedule of Accumulated Other Comprehensive Loss	The following is a summary of the changes in the components of AOCL, which are included in People’s United’s stockholders’ equity on an after-tax (in millions) Pension Net Unrealized Available-for-Sale Net Unrealized Held-to-Maturity Net Unrealized Total Balance at December 31, 2017 $ (144.1 ) $ (21.6 ) $ (15.1 ) $ (0.9 ) $ (181.7 ) Other comprehensive income (loss) before reclassifications — (56.2 ) — (2.4 ) (58.6 ) Amounts reclassified from AOCL (1) 4.5 (0.1 ) 2.3 0.1 6.8 Current period other comprehensive 4.5 (56.3 ) 2.3 (2.3 ) (51.8 ) Transition adjustments related to adoption of new accounting standards (2) (30.0 ) (3.9 ) (3.2 ) (0.2 ) (37.3 ) Balance at September 30, 2018 $ (169.6 ) $ (81.8 ) $ (16.0 ) $ (3.4 ) $ (270.8 ) (in millions) Pension and Net Unrealized Available-for-Sale Net Unrealized Held-to-Maturity Net Unrealized Total Balance at December 31, 2016 $ (145.6 ) $ (32.3 ) $ (17.4 ) $ 0.3 $ (195.0 ) Other comprehensive income before reclassifications — 11.1 — 0.3 11.4 Amounts reclassified from AOCL (1) 2.8 9.9 1.6 (0.4 ) 13.9 Current period other comprehensive 2.8 21.0 1.6 (0.1 ) 25.3 Balance at September 30, 2017 $ (142.8 ) $ (11.3 ) $ (15.8 ) $ 0.2 $ (169.7 ) (1) See the following table for details about these reclassifications. (2) See Note 13.
Summary of Amounts Reclassified from AOCL	The following is a summary of the amounts reclassified from AOCL: Amounts Reclassified from AOCL Affected Line Item Three Months Ended Nine Months Ended (in millions) 2018 2017 2018 2017 Details about components of AOCL: Amortization of pension and other postretirement plans items: Net actuarial loss $ (2.0 ) $ (1.7 ) $ (6.2 ) $ (5.1 ) (1) Prior service credit — 0.2 0.2 0.6 (1) (2.0 ) (1.5 ) (6.0 ) (4.5 ) Income before income tax expense 0.5 0.6 1.5 1.7 Income tax expense (1.5 ) (0.9 ) (4.5 ) (2.8 ) Net income Reclassification adjustment for net realized gains (losses) on debt securities available-for-sale — — 0.1 (15.6 ) Income before income tax expense (2) — — — 5.7 Income tax expense — — 0.1 (9.9 ) Net income Amortization of unrealized losses on debt securities transferred to held-to-maturity (0.9 ) (0.9 ) (3.0 ) (2.6 ) Income before income tax expense (3) 0.2 0.3 0.7 1.0 Income tax expense (0.7 ) (0.6 ) (2.3 ) (1.6 ) Net income Amortization of unrealized gains and losses on cash flow hedges: Interest rate swaps (0.3 ) 0.3 (0.3 ) 0.6 (4) Interest rate locks 0.1 0.1 0.1 0.1 (4) (0.2 ) 0.4 (0.2 ) 0.7 Income before income tax expense 0.1 (0.2 ) 0.1 (0.3 ) Income tax expense (0.1 ) 0.2 (0.1 ) 0.4 Net income Total reclassifications for the period $ (2.3 ) $ (1.3 ) $ (6.8 ) $ (13.9 ) (1) Included in the computation of net periodic benefit income (expense) reflected in other non-interest (2) Included in other non-interest (3) Included in interest and dividend income - securities. (4) Included in interest expense - notes and debentures.

Earnings Per Common Share (Tabl

Earnings Per Common Share (Tables)	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Basic and Diluted Earnings Per Common Share, Reflecting Application of Two-Class Method	The following is an analysis of People’s United’s basic and diluted earnings per common share (“EPS”), reflecting the application of the two-class Three Months Ended Nine Months Ended (in millions, except per common share data) 2018 2017 2018 2017 Net income available to common shareholders $ 113.5 $ 87.3 $ 324.6 $ 220.4 Dividends paid on and undistributed earnings allocated to participating securities — (0.1 ) (0.2 ) (0.4 ) Earnings attributable to common shareholders $ 113.5 $ 87.2 $ 324.4 $ 220.0 Weighted average common shares outstanding for basic EPS 341.4 336.9 340.6 327.6 Effect of dilutive equity-based awards 3.6 1.9 3.9 2.0 Weighted average common shares and common-equivalent shares for diluted EPS 345.0 338.8 344.5 329.6 EPS: Basic $ 0.33 $ 0.26 $ 0.95 $ 0.67 Diluted 0.33 0.26 0.94 0.67

Goodwill and Other Acquisitio_2

Goodwill and Other Acquisition-Related Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Changes in Carrying Amount of Goodwill	Changes in the carrying amounts of People’s United’s goodwill for the nine months ended September 30, 2018 and 2017 are summarized as follows: Operating Segment (in millions) Commercial Retail Wealth Total Balance at December 31, 2017 $ 1,600.3 $ 720.1 $ 91.0 $ 2,411.4 Acquisition of Vend Lease 23.8 — — 23.8 Balance at September 30, 2018 $ 1,624.1 $ 720.1 $ 91.0 $ 2,435.2 Operating Segment (in millions) Commercial Retail Wealth Total Balance at December 31, 2016 $ 1,222.1 $ 679.6 $ 91.0 $ 1,992.7 Acquisition of: Suffolk Bancorp 229.8 40.5 — 270.3 LEAF 148.4 — — 148.4 Balance at September 30, 2017 $ 1,600.3 $ 720.1 $ 91.0 $ 2,411.4

Employee Benefit Plans (Tables)

Employee Benefit Plans (Tables)	9 Months Ended
	Sep. 30, 2018
Retirement Benefits [Abstract]
Components of Net Periodic Benefit (Income) Expense and Other Amounts	Components of net periodic benefit (income) expense and other amounts recognized in other comprehensive income (loss) for the People’s Qualified Plan, the Chittenden Qualified Plan and the Supplemental Plans (together the “Pension Plans”) and the Other Postretirement Plan are as follows: Pension Plans Other Three months ended September 30 (in millions) 2018 2017 2018 2017 Net periodic benefit (income) expense: Interest cost $ 4.9 $ 4.9 $ 0.2 $ 0.2 Expected return on plan assets (10.9 ) (9.6 ) — — Recognized net actuarial loss 2.0 1.6 — 0.1 Recognized prior service credit — (0.2 ) — — Settlements (0.5 ) 1.1 — — Net periodic benefit (income) expense (1) $ (4.5 ) $ (2.2 ) $ 0.2 $ 0.3 Pension Plans Other Nine months ended September 30 (in millions) 2018 2017 2018 2017 Net periodic benefit (income) expense: Interest cost $ 14.6 $ 14.3 $ 0.6 $ 0.6 Expected return on plan assets (32.7 ) (28.3 ) — — Recognized net actuarial loss 6.0 4.9 0.2 0.2 Recognized prior service credit (0.2 ) (0.6 ) — — Settlements 0.4 2.6 — — Net periodic benefit (income) expense (1) (11.9 ) (7.1 ) 0.8 0.8 Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss): Net actuarial loss (6.0 ) (4.9 ) (0.2 ) (0.2 ) Prior service credit 0.2 0.6 — — Total pre-tax (5.8 ) (4.3 ) (0.2 ) (0.2 ) Total recognized in net periodic benefit (income) expense and other comprehensive income (loss) $ (17.7 ) $ (11.4 ) $ 0.6 $ 0.6 (1) As discussed in Note 13, amounts are included in other non-interest

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables summarize People’s United’s financial instruments that are measured at fair value on a recurring basis: Fair Value Measurements Using As of September 30, 2018 (in millions) Level 1 Level 2 Level 3 Total Financial assets: Trading debt securities: U.S. Treasury $ 8.3 $ — $ — $ 8.3 Debt securities available-for-sale: U.S. Treasury and agency 652.1 — — 652.1 GSE mortgage-backed securities — 2,660.0 — 2,660.0 Equity securities 8.9 — — 8.9 Other assets: Exchange-traded funds 39.1 — — 39.1 Mutual funds 3.1 — — 3.1 Fixed income securities — 0.3 — 0.3 Interest rate swaps — 45.9 — 45.9 Interest rate caps — 3.6 — 3.6 Foreign exchange contracts — 0.6 — 0.6 Forward commitments to sell residential mortgage loans — 0.2 — 0.2 Total $ 711.5 $ 2,710.6 $ — $ 3,422.1 Financial liabilities: Interest rate swaps $ — $ 207.4 $ — $ 207.4 Interest rate caps — 3.6 — 3.6 Risk participation agreements (1) — — — — Foreign exchange contracts — 0.4 — 0.4 Interest rate-lock commitments on residential mortgage loans — 0.2 — 0.2 Total $ — $ 211.6 $ — $ 211.6 Fair Value Measurements Using As of December 31, 2017 (in millions) Level 1 Level 2 Level 3 Total Financial assets: Trading debt securities: U.S. Treasury $ 8.2 $ — $ — $ 8.2 Debt securities available-for-sale: U.S. Treasury and agency 668.8 — — 668.8 GSE mortgage-backed securities — 2,456.5 — 2,456.5 Equity securities 8.7 — — 8.7 Other assets: Exchange-traded funds 36.5 — — 36.5 Mutual funds 3.5 — — 3.5 Fixed income securities — 1.3 — 1.3 Interest rate swaps — 74.8 — 74.8 Interest rate caps — 2.8 — 2.8 Foreign exchange contracts — 0.1 — 0.1 Forward commitments to sell residential mortgage loans — 0.2 — 0.2 Total $ 725.7 $ 2,535.7 $ — $ 3,261.4 Financial liabilities: Interest rate swaps $ — $ 84.9 $ — $ 84.9 Interest rate caps — 2.8 — 2.8 Risk participation agreements (1) — — — — Foreign exchange contracts — 0.4 — 0.4 Interest rate-lock commitments on residential mortgage loans — 0.2 — 0.2 Total $ — $ 88.3 $ — $ 88.3 (1) At both September 30, 2018 and December 31, 2017, the fair value of risk participation agreements totaled less than $0.1 million (see Note 11).
Assets Measured at Fair Value on Non-Recurring Basis	The following tables summarize People’s United’s assets that are measured at fair value on a non-recurring Fair Value Measurements Using As of September 30, 2018 (in millions) Level 1 Level 2 Level 3 Total Loans held-for-sale $ — $ 15.2 $ — $ 15.2 Impaired loans (2) — — 54.1 54.1 REO and repossessed assets (3) — — 15.1 15.1 Total $ — $ 15.2 $ 69.2 $ 84.4 Fair Value Measurements Using As of December 31, 2017 (in millions) Level 1 Level 2 Level 3 Total Loans held-for-sale $ — $ 16.6 $ — $ 16.6 Impaired loans (2) — — 59.6 59.6 REO and repossessed assets (3) — — 19.4 19.4 Total $ — $ 16.6 $ 79.0 $ 95.6 (1) Consists of residential mortgage loans; no fair value adjustments were recorded for the nine months ended September 30, 2018 and 2017. (2) Represents the recorded investment in originated impaired loans with a related allowance for loan losses measured in accordance with applicable accounting guidance. The total consists of $42.0 million of Commercial loans and $12.1 million of Retail loans at September 30, 2018. The provision for loan losses on impaired loans totaled $7.7 million and $4.1 million for the nine months ended September 30, 2018 and 2017, respectively. (3) Represents: (i) $8.7 million of commercial REO; (ii) $4.4 million of residential REO; and (iii) $2.0 million of repossessed assets at September 30, 2018. Charge-offs to the allowance for loan losses related to loans that were transferred to REO or repossessed assets totaled $1.0 million and $2.2 million for the nine months ended September 30, 2018 and 2017, respectively. Write downs and net loss on sale of foreclosed/repossessed assets charged to non-interest
Carrying Amounts and Estimated Fair Values of Financial Instruments	The following tables summarize the carrying amounts, estimated fair values and placement in the fair value hierarchy of People’s United’s financial instruments that are not measured at fair value either on a recurring or non-recurring Estimated Fair Value Carrying Measurements Using As of September 30, 2018 (in millions) Level 1 Level 2 Level 3 Total Financial assets: Cash and due from banks $ 410.5 $ 410.5 $ — $ — $ 410.5 Short-term investments 127.5 — 127.5 — 127.5 Debt securities held-to-maturity 3,742.9 — 3,665.3 1.5 3,666.8 FHLB and FRB stock 312.4 — 312.4 — 312.4 Total loans, net (1) 31,907.2 — 6,582.7 24,805.5 31,388.2 Financial liabilities: Time deposits 6,035.9 — 5,997.6 — 5,997.6 Other deposits 27,174.3 — 27,174.3 — 27,174.3 FHLB advances 2,369.7 — 2,370.2 — 2,370.2 Federal funds purchased 735.0 — 735.0 — 735.0 Customer repurchase agreements 261.3 — 261.3 — 261.3 Other borrowings 26.0 — 26.0 — 26.0 Notes and debentures 885.6 — 892.5 — 892.5 (1) Excludes impaired loans totaling $54.1 million measured at fair value on a non-recurring Estimated Fair Value Carrying Measurements Using As of December 31, 2017 (in millions) Level 1 Level 2 Level 3 Total Financial assets: Cash and due from banks $ 505.1 $ 505.1 $ — $ — $ 505.1 Short-term investments 377.5 — 377.5 — 377.5 Debt securities held-to-maturity 3,588.1 — 3,632.2 1.5 3,633.7 FHLB and FRB stock 312.3 — 312.3 — 312.3 Total loans, net (1) 32,281.3 — 6,632.2 25,495.3 32,127.5 Financial liabilities: Time deposits 5,454.3 — 5,441.1 — 5,441.1 Other deposits 27,602.0 — 27,602.0 — 27,602.0 FHLB advances 2,774.4 — 2,775.3 — 2,775.3 Federal funds purchased 820.0 — 820.0 — 820.0 Customer repurchase agreements 301.6 — 301.6 — 301.6 Other borrowings 207.8 — 207.2 — 207.2 Notes and debentures 901.6 — 910.1 — 910.1 (1) Excludes impaired loans totaling $59.6 million measured at fair value on a non-recurring

Derivative Financial Instrume_2

Derivative Financial Instruments and Hedging Activities (Tables)	9 Months Ended
	Sep. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Notional Amounts and Fair Values of Derivatives Outstanding	The table below provides a summary of the notional amounts and fair values (presented on a gross basis) of derivatives outstanding: Fair Values (1) Notional Amounts Assets Liabilities (in millions) Type of Sept. 30, Dec. 31, Sept. 30, Dec. 31, Sept. 30, Dec. 31, Derivatives Not Designated as Hedging Instruments: Interest rate swaps: Commercial customers N/A $ 6,576.2 $ 5,769.1 $ 17.5 $ 64.7 $ 199.1 $ 61.2 Institutional counterparties N/A 6,282.0 5,775.9 28.4 10.1 8.3 23.7 Interest rate caps: Commercial customers N/A 313.0 649.2 — — 3.6 2.8 Institutional counterparties N/A 313.0 649.2 3.6 2.8 — — Risk participation agreements (2) N/A 455.4 439.4 — — — — Foreign exchange contracts N/A 99.2 46.5 0.6 0.1 0.4 0.4 Forward commitments to sell N/A 18.9 16.4 0.2 0.2 — — Interest rate-lock commitments on residential mortgage loans N/A 24.0 18.3 — — 0.2 0.2 Total 50.3 77.9 211.6 88.3 Derivatives Designated as Hedging Instruments: Interest rate swaps: Subordinated notes Fair value 375.0 375.0 — — — — Loans Cash flow 210.0 210.0 — — — — Total — — — — Total fair value of derivatives $ 50.3 $ 77.9 $ 211.6 $ 88.3 (1) Assets are recorded in other assets and liabilities are recorded in other liabilities. (2) Fair value totaled less than $0.1 million at both dates.
Impact of Derivatives on Pre-Tax Income and Accumulated Other Comprehensive Loss	The following table summarizes the impact of People’s United’s derivatives on pre-tax Amount of Pre-Tax Gain (Loss) Amount of Pre-Tax Type of Recognized in Earnings (1) Recognized in AOCL Nine months ended September 30 (in millions) 2018 2017 2018 2017 Derivatives Not Designated as Hedging Instruments: Interest rate swaps: Commercial customers N/A $ (177.1 ) $ 44.2 $ — $ — Institutional counterparties N/A 184.5 (38.4 ) — — Interest rate caps: Commercial customers N/A (0.7 ) 1.2 — — Institutional counterparties N/A 0.8 (0.7 ) — — Foreign exchange contracts N/A 0.6 0.3 — — Risk participation agreements N/A 0.2 0.5 — — Forward commitments to sell residential mortgage loans N/A — 0.1 — — Interest rate-lock commitments on residential mortgage loans N/A — (0.1 ) — — Total 8.3 7.1 — — Derivatives Designated as Hedging Instruments: Interest rate swaps Fair value (1.8 ) 4.5 — — Interest rate swaps Cash flow (0.3 ) 0.6 (3.1 ) 0.4 Interest rate locks Cash flow 0.1 0.1 — — Total (2.0 ) 5.2 (3.1 ) 0.4 Total $ 6.3 $ 12.3 $ (3.1 ) $ 0.4 (1) Amounts recognized in earnings are recorded in interest income, interest expense or other non-interest non-interest

Balance Sheet Offsetting (Table

Balance Sheet Offsetting (Tables)	9 Months Ended
	Sep. 30, 2018
Text Block [Abstract]
Summary of Gross Presentation, Financial Instruments that are Eligible for Offset Not Offset in Consolidated Statement of Condition	The following tables provide a gross presentation, the effects of offsetting, and a net presentation of the Company’s financial instruments that are eligible for offset in the Consolidated Statements of Condition. The collateral amounts in these tables are limited to the outstanding balances of the related asset or liability (after netting is applied) and, therefore, instances of overcollateralization are not presented. In the tables below, the Net Amount Presented of the derivative assets and liabilities can be reconciled to the fair value of the Company’s derivative financial instruments in Note 11. The Company’s derivative contracts with commercial customers and customer repurchase agreements are not subject to master netting arrangements and, therefore, have been excluded from the tables below. Gross Gross Net Gross Amounts Not Offset Financial Net As of September 30, 2018 (in millions) Instruments Collateral Amount Financial assets: Interest rate swaps: Counterparty A $ 5.1 $ — $ 5.1 $ (1.0 ) $ (4.1 ) $ — Counterparty B 4.4 — 4.4 (2.7 ) (1.7 ) — Counterparty C 10.3 — 10.3 (1.2 ) (9.1 ) — Counterparty D 6.3 — 6.3 (1.4 ) (4.9 ) — Counterparty E — — — — — — Other counterparties 5.9 — 5.9 (0.1 ) (5.7 ) 0.1 Foreign exchange contracts 0.6 — 0.6 — — 0.6 Total $ 32.6 $ — $ 32.6 $ (6.4 ) $ (25.5 ) $ 0.7 Financial liabilities: Interest rate swaps: Counterparty A $ 1.0 $ — $ 1.0 $ (1.0 ) $ — $ — Counterparty B 2.7 — 2.7 (2.7 ) — — Counterparty C 1.2 — 1.2 (1.2 ) — — Counterparty D 1.4 — 1.4 (1.4 ) — — Counterparty E 1.9 — 1.9 — — 1.9 Other counterparties 0.1 — 0.1 (0.1 ) — — Foreign exchange contracts 0.4 — 0.4 — — 0.4 Total $ 8.7 $ — $ 8.7 $ (6.4 ) $ — $ 2.3 Gross Gross Net Gross Amounts Not Offset Financial Net As of December 31, 2017 (in millions) Instruments Collateral Amount Financial assets: Interest rate swaps: Counterparty A $ 2.6 $ — $ 2.6 $ (2.5 ) $ — $ 0.1 Counterparty B 1.6 — 1.6 (1.6 ) — — Counterparty C 2.6 — 2.6 (2.6 ) — — Counterparty D 3.5 — 3.5 (3.5 ) — — Counterparty E — — — — — — Other counterparties 2.6 — 2.6 (0.2 ) (2.4 ) — Foreign exchange contracts 0.1 — 0.1 — — 0.1 Total $ 13.0 $ — $ 13.0 $ (10.4 ) $ (2.4 ) $ 0.2 Financial liabilities: Interest rate swaps: Counterparty A $ 2.5 $ — $ 2.5 $ (2.5 ) $ — $ — Counterparty B 5.6 — 5.6 (1.6 ) (4.0 ) — Counterparty C 2.8 — 2.8 (2.6 ) (0.2 ) — Counterparty D 4.7 — 4.7 (3.5 ) (1.2 ) — Counterparty E 7.3 — 7.3 — — 7.3 Other counterparties 0.8 — 0.8 (0.2 ) (0.6 ) — Foreign exchange contracts 0.4 — 0.4 — — 0.4 Total $ 24.1 $ — $ 24.1 $ (10.4 ) $ (6.0 ) $ 7.7
Summary of Collateral Swaps	The following tables show the extent to which assets and liabilities exchanged under resale and repurchase agreements have been offset in the Consolidated Statements of Condition. These agreements: (i) are entered into simultaneously with the same financial institution counterparty; (ii) have the same principal amounts and inception/maturity dates; and (iii) are subject to a master netting arrangement that contains a conditional right of offset upon default. At September 30, 2018 and December 31, 2017, the Company posted as collateral marketable securities with fair values of $463.9 million and $453.8 million, respectively, and, in turn, accepted as collateral marketable securities with fair values of $453.8 million and $461.9 million, respectively. As of September 30, 2018 (in millions) Gross Gross Net Total resale agreements $ 450.0 $ (450.0 ) $ — Total repurchase agreements $ 450.0 $ (450.0 ) $ — As of December 31, 2017 (in millions) Gross Gross Net Total resale agreements $ 450.0 $ (450.0 ) $ — Total repurchase agreements $ 450.0 $ (450.0 ) $ —

General - Additional Informatio

General - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 27, 2018	Dec. 31, 2017
Description of Business and Summary of Significant Accounting Policies [Line Items]
Affordable housing investments	$ 286.6		$ 286.6			$ 250.7
Future contingent commitments	112.5		$ 112.5			$ 99.6
Amortization period			10 years
Income tax expense	$ 4.8	$ 4.2	$ 14	$ 12.3
Merger related expenses			$ 1.9
Vend Lease [Member]
Description of Business and Summary of Significant Accounting Policies [Line Items]
Payment to acquire lease and loan portfolio					$ 68

Cash and Cash Equivalents and_3

Cash and Cash Equivalents and Securities - Additional Information (Detail)	1 Months Ended	3 Months Ended			9 Months Ended		12 Months Ended
	Jan. 31, 2018USD ($)	Sep. 30, 2018USD ($)SecuritySecurities	Sep. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Sep. 30, 2018USD ($)SecuritySecurities	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)
Investment [Line Items]
Restricted cash		$ 0			$ 0		$ 13,800,000
Debt security available for sale with a fair value pledged with collateral for public deposits		1,870,000,000			1,870,000,000		1,940,000,000
Debt security available for sale with a held-to-maturity pledged with collateral for public deposits		$ 1,300,000,000			$ 1,300,000,000		1,370,000,000
Number of debt securities classified as available-for-sale | Security		151			151
Number of debt securities classified as held-to-maturity | Securities		1,042			1,042
Number of debt securities owned | Security					2,137
Percentage of securities available for sale		56.00%			56.00%
Available for sale of securities gross unrealized losses		$ 115,300,000			$ 115,300,000
Held to maturity securities gross unrealized losses		95,800,000			95,800,000		20,000,000
Impairment losses on securities recognized in earnings		0	$ 0		0	$ 0
Amortized cost of mortgage-backed securities		3,426,000,000			3,426,000,000		3,164,900,000
Gain (loss) on sales of Mortgage Backed Securities		200,000			1,000,000
Equity securities, at fair value		8,900,000			8,900,000		8,700,000
Impairment of investments					$ 0
Federal Reserve Bank stock percentage					6.00%
Federal Reserve Bank of New York [Member]
Investment [Line Items]
Interest-bearing deposits		107,800,000			$ 107,800,000		$ 340,400,000
Federal funds sold and yield					2.25%		1.50%
Acquired shares of capital stock		177,600,000			$ 177,600,000		$ 170,300,000
FHLB of New York [Member]
Investment [Line Items]
Acquired shares of capital stock		700,000			700,000		12,000,000
FHLB of Boston [Member]
Investment [Line Items]
Acquired shares of capital stock		134,100,000			134,100,000		130,000,000
Accounting Standards Update 2016-01 [Member]
Investment [Line Items]
Equity securities with an amortized cost							9,600,000
Equity securities, at fair value							8,700,000
Cumulative-effect transition adjustment to retained earnings, net of tax	$ (600,000)						(600,000)
GSE Mortgage-Backed Securities [Member]
Investment [Line Items]
Held to maturity securities gross unrealized losses		57,700,000			$ 57,700,000		15,400,000
Available for sale securities average maturity period					9 years
Amortized cost of mortgage-backed securities		2,742,500,000			$ 2,742,500,000		2,477,800,000
State and Municipal [Member]
Investment [Line Items]
Held to maturity securities gross unrealized losses		37,400,000			$ 37,400,000		$ 4,600,000
Available for sale securities average maturity period					12 years
U.S. Treasury and CMO Securities [Member]
Investment [Line Items]
Amortized cost of mortgage-backed securities				$ 486,900,000
Gain (loss) on sales of Mortgage Backed Securities				$ 15,700,000
Maximum [Member]
Investment [Line Items]
Debt securities with unrealized losses		$ 100,000			$ 100,000

Cash and Cash Equivalents and_4

Cash and Cash Equivalents and Securities - Available-for-Sale and Held-to-Maturity Debt Securities Gains (Losses) (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Amortized Cost	$ 3,426	$ 3,164.9
Gross Unrealized Gains	1.4	7.4
Gross Unrealized Losses	(115.3)	(47)
Fair Value	3,312.1	3,125.3
Held to maturity Securities, Amortized Cost	3,742.9	3,588.1
Securities held to maturity, Gross Unrealized Gains	19.7	65.6
Securities held to maturity, Gross Unrealized Losses	(95.8)	(20)
Total securities held to maturity, Fair Value	3,666.8	3,633.7
State and Municipal [Member]
Investment Holdings [Line Items]
Held to maturity Securities, Amortized Cost	2,268.2	2,060.4
Securities held to maturity, Gross Unrealized Gains	19.7	64.5
Securities held to maturity, Gross Unrealized Losses	(37.4)	(4.6)
Total securities held to maturity, Fair Value	2,250.5	2,120.3
GSE Mortgage-Backed Securities [Member]
Investment Holdings [Line Items]
Amortized Cost	2,742.5	2,477.8
Gross Unrealized Gains	1.4	7.4
Gross Unrealized Losses	(83.9)	(28.7)
Fair Value	2,660	2,456.5
Held to maturity Securities, Amortized Cost	1,407.3	1,474.9
Securities held to maturity, Gross Unrealized Gains		0.3
Securities held to maturity, Gross Unrealized Losses	(57.7)	(15.4)
Total securities held to maturity, Fair Value	1,349.6	1,459.8
U.S. Treasury and Agency [Member]
Investment Holdings [Line Items]
Amortized Cost	683.5	687.1
Gross Unrealized Losses	(31.4)	(18.3)
Fair Value	652.1	668.8
Corporate [Member]
Investment Holdings [Line Items]
Held to maturity Securities, Amortized Cost	65.9	51.3
Securities held to maturity, Gross Unrealized Gains		0.8
Securities held to maturity, Gross Unrealized Losses	(0.7)
Total securities held to maturity, Fair Value	65.2	52.1
Other [Member]
Investment Holdings [Line Items]
Held to maturity Securities, Amortized Cost	1.5	1.5
Total securities held to maturity, Fair Value	$ 1.5	$ 1.5

Cash and Cash Equivalents and_5

Cash and Cash Equivalents and Securities - Summary of Amortized Cost and Fair Value of Debt Securities Based on Remaining Period to Contractual Maturity (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Available for Sale, Amortized Cost, Within 1 year	$ 6
Available for Sale, Amortized Cost, After 1 but within 5 years	731.1
Available for Sale, Amortized Cost, After 5 but within 10 years	1,014.6
Available for Sale, Amortized Cost, After 10 years	1,674.3
Amortized Cost	3,426	$ 3,164.9
Available for Sale, Fair Value, Within 1 year	6
Available for Sale, Fair Value, After 1 but within 5 years	699
Available for Sale, Fair Value, After 5 but within 10 years	1,000.8
Available for Sale, Fair Value, After 10 years	1,606.3
Available for Sale, Fair Value, Total	3,312.1	3,125.3
Held to Maturity, Amortized Cost, Within 1 year	27.8
Held to Maturity, Amortized Cost, After 1 but within 5 years	421.5
Held to Maturity, Amortized Cost, After 5 but within 10 years	1,195.7
Held to Maturity, Amortized Cost, After 10 years	2,097.9
Held to maturity Securities, Amortized Cost	3,742.9	3,588.1
Held to Maturity, Fair Value, Within 1 year	27.8
Held to Maturity, Fair Value, After 1 but within 5 years	416.1
Held to Maturity, Fair Value, After 5 but within 10 years	1,165.2
Held to Maturity, Fair Value, After 10 years	2,057.7
Held to Maturity, Fair Value, Total	3,666.8	3,633.7
U.S. Treasury and Agency [Member]
Investment [Line Items]
Available for Sale, Amortized Cost, Within 1 year	6
Available for Sale, Amortized Cost, After 1 but within 5 years	677.5
Amortized Cost	683.5	687.1
Available for Sale, Fair Value, Within 1 year	6
Available for Sale, Fair Value, After 1 but within 5 years	646.1
Available for Sale, Fair Value, Total	652.1	668.8
GSE Mortgage-Backed Securities [Member]
Investment [Line Items]
Available for Sale, Amortized Cost, After 1 but within 5 years	53.6
Available for Sale, Amortized Cost, After 5 but within 10 years	1,014.6
Available for Sale, Amortized Cost, After 10 years	1,674.3
Amortized Cost	2,742.5	2,477.8
Available for Sale, Fair Value, After 1 but within 5 years	52.9
Available for Sale, Fair Value, After 5 but within 10 years	1,000.8
Available for Sale, Fair Value, After 10 years	1,606.3
Available for Sale, Fair Value, Total	2,660	2,456.5
Held to Maturity, Amortized Cost, Within 1 year	4.3
Held to Maturity, Amortized Cost, After 1 but within 5 years	234.7
Held to Maturity, Amortized Cost, After 5 but within 10 years	800.4
Held to Maturity, Amortized Cost, After 10 years	367.9
Held to maturity Securities, Amortized Cost	1,407.3	1,474.9
Held to Maturity, Fair Value, Within 1 year	4.3
Held to Maturity, Fair Value, After 1 but within 5 years	226.4
Held to Maturity, Fair Value, After 5 but within 10 years	764.9
Held to Maturity, Fair Value, After 10 years	354
Held to Maturity, Fair Value, Total	1,349.6	1,459.8
State and Municipal [Member]
Investment [Line Items]
Held to Maturity, Amortized Cost, Within 1 year	23.5
Held to Maturity, Amortized Cost, After 1 but within 5 years	180.3
Held to Maturity, Amortized Cost, After 5 but within 10 years	334.4
Held to Maturity, Amortized Cost, After 10 years	1,730
Held to maturity Securities, Amortized Cost	2,268.2	2,060.4
Held to Maturity, Fair Value, Within 1 year	23.5
Held to Maturity, Fair Value, After 1 but within 5 years	183.2
Held to Maturity, Fair Value, After 5 but within 10 years	340.1
Held to Maturity, Fair Value, After 10 years	1,703.7
Held to Maturity, Fair Value, Total	2,250.5	2,120.3
Corporate [Member]
Investment [Line Items]
Held to Maturity, Amortized Cost, After 1 but within 5 years	5
Held to Maturity, Amortized Cost, After 5 but within 10 years	60.9
Held to maturity Securities, Amortized Cost	65.9	51.3
Held to Maturity, Fair Value, After 1 but within 5 years	5
Held to Maturity, Fair Value, After 5 but within 10 years	60.2
Held to Maturity, Fair Value, Total	65.2	52.1
Other [Member]
Investment [Line Items]
Held to Maturity, Amortized Cost, After 1 but within 5 years	1.5
Held to maturity Securities, Amortized Cost	1.5	1.5
Held to Maturity, Fair Value, After 1 but within 5 years	1.5
Held to Maturity, Fair Value, Total	$ 1.5	$ 1.5

Cash and Cash Equivalents and_6

Cash and Cash Equivalents and Securities - Continuous Unrealized Loss Position on Available-for-Sale and Held-to-Maturities Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Continuous Unrealized Loss Position, Less Than 12 Months, Fair Value	$ 2,476.3	$ 2,565
Continuous Unrealized Loss Position, Less Than 12 Months, Unrealized Losses	(50.2)	(23.9)
Continuous Unrealized Loss Position, 12 Months Or Longer, Fair Value	3,383.6	1,892.4
Continuous Unrealized Loss Position, 12 Months Or Longer, Unrealized Losses	(160.9)	(43.1)
Continuous Unrealized Loss Position, Total, Fair Value	5,859.9	4,457.4
Continuous Unrealized Loss Position, Total, Unrealized Losses	(211.1)	(67)
GSE Mortgage-Backed Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Held To Maturity Securities, Continuous Unrealized Loss Position, Less Than 12 Months, Fair Value	330.7	1,289.3
Held To Maturity Securities, Continuous Unrealized Loss Position, Less Than 12 Months, Unrealized Losses	(11.9)	(14.7)
Held To Maturity Securities, Continuous Unrealized Loss Position, 12 Months Or Longer, Fair Value	1,018.9	45
Held To Maturity Securities, Continuous Unrealized Loss Position, 12 Months Or Longer, Unrealized Losses	(45.8)	(0.7)
Held To Maturity Securities, Continuous Unrealized Loss Position, Total, Fair Value	1,349.6	1,334.3
Held To Maturity Securities, Continuous Unrealized Loss Position, Total, Unrealized Losses	(57.7)	(15.4)
Available for Sale Securities, Continuous Unrealized Loss Position, Less Than 12 Months, Fair Value	893.6	1,013.5
Available for Sale Securities, Continuous Unrealized Loss Position, Less Than 12 Months, Unrealized Losses	(14.2)	(8.7)
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months Or Longer, Fair Value	1,604.7	1,114.8
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months Or Longer, Unrealized Losses	(69.7)	(20)
Available for Sale Securities, Continuous Unrealized Loss Position, Total, Fair Value	2,498.3	2,128.3
Available for Sale Securities, Continuous Unrealized Loss Position, Total, Unrealized Losses	(83.9)	(28.7)
U.S. Treasury and Agency [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available for Sale Securities, Continuous Unrealized Loss Position, Less Than 12 Months, Fair Value	152.5	156
Available for Sale Securities, Continuous Unrealized Loss Position, Less Than 12 Months, Unrealized Losses	(1.8)
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months Or Longer, Fair Value	494.6	507.7
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months Or Longer, Unrealized Losses	(29.6)	(18.3)
Available for Sale Securities, Continuous Unrealized Loss Position, Total, Fair Value	647.1	663.7
Available for Sale Securities, Continuous Unrealized Loss Position, Total, Unrealized Losses	(31.4)	(18.3)
State and Municipal [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Held To Maturity Securities, Continuous Unrealized Loss Position, Less Than 12 Months, Fair Value	1,048.1	106.2
Held To Maturity Securities, Continuous Unrealized Loss Position, Less Than 12 Months, Unrealized Losses	(21.6)	(0.5)
Held To Maturity Securities, Continuous Unrealized Loss Position, 12 Months Or Longer, Fair Value	265.4	224.9
Held To Maturity Securities, Continuous Unrealized Loss Position, 12 Months Or Longer, Unrealized Losses	(15.8)	(4.1)
Held To Maturity Securities, Continuous Unrealized Loss Position, Total, Fair Value	1,313.5	331.1
Held To Maturity Securities, Continuous Unrealized Loss Position, Total, Unrealized Losses	(37.4)	$ (4.6)
Corporate [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Held To Maturity Securities, Continuous Unrealized Loss Position, Less Than 12 Months, Fair Value	51.4
Held To Maturity Securities, Continuous Unrealized Loss Position, Less Than 12 Months, Unrealized Losses	(0.7)
Held To Maturity Securities, Continuous Unrealized Loss Position, Total, Fair Value	51.4
Held To Maturity Securities, Continuous Unrealized Loss Position, Total, Unrealized Losses	$ (0.7)

Loans - Additional Information

Loans - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)SegmentsRating	Sep. 30, 2017USD ($)	Jun. 27, 2018USD ($)	Dec. 31, 2017USD ($)	Jun. 30, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2011USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of segments | Segments			2
Net deferred loan costs	$ 91,700,000		$ 91,700,000			$ 80,400,000
Recorded investments, non-performing loans	158,100,000		158,100,000			148,700,000
Accruing interest total	0		$ 0			0
Number of days of non accrual status			90 days
Allowance for loan losses	238,000,000	$ 233,400,000	$ 238,000,000	$ 233,400,000	$ 236,800,000	234,400,000	$ 231,600,000	$ 229,300,000
Interest Income Recognized	1,300,000	1,700,000	$ 4,100,000	4,500,000
Temporary reduction of interest rate for TDRs, basis points			2.00%
Collateral values/LTV ratios, minimum			70.00%
Borrower credit scores, minimum | Rating			680
Principal and interest payments receivable									$ 7,650,000,000
Expected cash flows									7,090,000,000
Loan portfolio fair value									5,420,000,000
Aggregate loan nonaccretable difference									560,100,000
Portfolio accretable yield	184,600,000	226,300,000	$ 184,600,000	226,300,000	190,200,000	219,700,000	233,400,000	255,400,000
Portfolio outstanding balance	476,400,000		476,400,000			587,700,000
Portfolio carrying value	394,700,000		394,700,000			498,500,000
Repossessed assets	2,000,000		2,000,000			2,500,000
Commercial Real Estate Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Early non-performing loans	8,600,000		8,600,000			10,600,000
Real estate owned	8,700,000		8,700,000			9,300,000
Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Early non-performing loans	31,800,000		31,800,000			19,500,000
Equipment Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Early non-performing loans	33,800,000		33,800,000			38,600,000
Residential Mortgage Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real estate owned	4,400,000		4,400,000			7,600,000
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans	111,400,000		111,400,000			100,600,000
Allowance for loan losses	207,100,000	203,500,000	207,100,000	203,500,000	205,800,000	204,500,000	201,900,000	204,900,000
Interest Income Recognized	700,000	1,200,000	2,400,000	2,900,000
Commercial [Member] | Commercial Real Estate Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans	17,200,000		17,200,000			23,700,000
Interest Income Recognized	400,000	300,000	900,000	900,000
Commercial [Member] | Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans	44,900,000		44,900,000			32,600,000
Interest Income Recognized	200,000	700,000	1,300,000	1,600,000
Commercial [Member] | Equipment Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans	49,300,000		49,300,000			44,300,000
Interest Income Recognized	100,000	200,000	200,000	400,000
Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans	46,700,000		46,700,000			48,100,000
Allowance for loan losses	30,900,000	29,900,000	30,900,000	29,900,000	$ 31,000,000	29,900,000	$ 29,700,000	$ 24,400,000
Interest Income Recognized	600,000	500,000	$ 1,700,000	1,600,000
Duration of extension for payment deferral on TDRs, years			5 years
Retail Loans [Member] | Residential Mortgage Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans	32,000,000		$ 32,000,000			32,700,000
Interest Income Recognized	400,000	400,000	$ 1,300,000	1,300,000
Maximum [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Duration of extension for payment deferral on TDRs, years			2 years
Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan agreement			6 months
At Acquisition [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Portfolio accretable yield									$ 1,670,000,000
Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans	23,700,000		$ 23,700,000			25,100,000
Acquired Loans [Member] | Purchased Performing Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans	8,600,000		8,600,000			4,600,000
Acquired Loans [Member] | Purchased Performing Loans [Member] | Subsequent to Acquisition [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans	8,600,000		8,600,000			4,600,000
Troubled Debt Restructurings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investment in originated loans classified as TDRs	177,400,000		177,400,000			186,900,000
Allowance for loan losses	5,400,000		5,400,000			4,400,000
Interest Income Recognized	1,200,000	$ 1,400,000	4,000,000	$ 3,700,000
Foreclosure or Bankruptcy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Early non-performing loans	$ 19,300,000		$ 19,300,000			$ 20,200,000

Loans - Summary of Loans by Loa

Loans - Summary of Loans by Loan Portfolio Segment and Class (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 32,199.3	$ 32,575.3
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	23,373.4	23,705.2
Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	8,825.9	8,870.1
Commercial Real Estate Loan [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	10,595.5	11,068.7
Commercial and Industrial [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	8,568.6	8,731.1
Equipment Financing [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	4,209.3	3,905.4
Residential Mortgage Adjustable Rate [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	5,937.5	5,926.6
Residential Mortgage Fixed Rate [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	974.4	879.1
Residential Mortgage Loan [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	6,911.9	6,805.7
Home Equity Loan [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,867.2	2,015.2
Other Consumer [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	46.8	49.2
Home Equity and Other Consumer [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,914	2,064.4
Originated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	30,110.8	30,111.2
Originated [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	21,564.7	21,565
Originated [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	8,546.1	8,546.2
Originated [Member] | Commercial Real Estate Loan [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	9,750.9	10,126.6
Originated [Member] | Commercial and Industrial [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	8,078.9	8,129.9
Originated [Member] | Equipment Financing [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	3,734.9	3,308.5
Originated [Member] | Residential Mortgage Adjustable Rate [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	5,810.8	5,782.6
Originated [Member] | Residential Mortgage Fixed Rate [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	867.5	758
Originated [Member] | Residential Mortgage Loan [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	6,678.3	6,540.6
Originated [Member] | Home Equity Loan [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,823.6	1,960
Originated [Member] | Other Consumer [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	44.2	45.6
Originated [Member] | Home Equity and Other Consumer [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,867.8	2,005.6
Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	2,088.5	2,464.1
Acquired Loans [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,808.7	2,140.2
Acquired Loans [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	279.8	323.9
Acquired Loans [Member] | Commercial Real Estate Loan [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	844.6	942.1
Acquired Loans [Member] | Commercial and Industrial [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	489.7	601.2
Acquired Loans [Member] | Equipment Financing [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	474.4	596.9
Acquired Loans [Member] | Residential Mortgage Adjustable Rate [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	126.7	144
Acquired Loans [Member] | Residential Mortgage Fixed Rate [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	106.9	121.1
Acquired Loans [Member] | Residential Mortgage Loan [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	233.6	265.1
Acquired Loans [Member] | Home Equity Loan [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	43.6	55.2
Acquired Loans [Member] | Other Consumer [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	2.6	3.6
Acquired Loans [Member] | Home Equity and Other Consumer [Member] | Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 46.2	$ 58.8

Loans - Summary, by Loan Portfo

Loans - Summary, by Loan Portfolio Segment, of Activity in Allowance for Loan Losses (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	$ 236.8	$ 231.6	$ 234.4	$ 229.3
Charge-offs	(8.4)	(6.8)	(21.8)	(20)
Recoveries	1.4	1.6	5.3	5.6
Net loan charge-offs	(7)	(5.2)	(16.5)	(14.4)
Provision for loan losses	8.2	7	20.1	18.5
Balance at end of period	238	233.4	238	233.4
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	205.8	201.9	204.5	204.9
Charge-offs	(7.7)	(5)	(19.2)	(14.9)
Recoveries	1	1	3.6	3.9
Net loan charge-offs	(6.7)	(4)	(15.6)	(11)
Provision for loan losses	8	5.6	18.2	9.6
Balance at end of period	207.1	203.5	207.1	203.5
Commercial [Member] | Originated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	202	198.3	201.1	198.8
Charge-offs	(5.7)	(4)	(12.9)	(12)
Recoveries	0.6	0.9	2.6	3.8
Net loan charge-offs	(5.1)	(3.1)	(10.3)	(8.2)
Provision for loan losses	6.3	4.3	12.4	8.9
Balance at end of period	203.2	199.5	203.2	199.5
Commercial [Member] | Acquired Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	3.8	3.6	3.4	6.1
Charge-offs	(2)	(1)	(6.3)	(2.9)
Recoveries	0.4	0.1	1	0.1
Net loan charge-offs	(1.6)	(0.9)	(5.3)	(2.8)
Provision for loan losses	1.7	1.3	5.8	0.7
Balance at end of period	3.9	4	3.9	4
Retail Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	31	29.7	29.9	24.4
Charge-offs	(0.7)	(1.8)	(2.6)	(5.1)
Recoveries	0.4	0.6	1.7	1.7
Net loan charge-offs	(0.3)	(1.2)	(0.9)	(3.4)
Provision for loan losses	0.2	1.4	1.9	8.9
Balance at end of period	30.9	29.9	30.9	29.9
Retail Loans [Member] | Originated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	30.8	29.6	29.7	24.2
Charge-offs	(0.7)	(1.8)	(2.6)	(5.1)
Recoveries	0.4	0.6	1.7	1.7
Net loan charge-offs	(0.3)	(1.2)	(0.9)	(3.4)
Provision for loan losses	0.2	1.3	1.9	8.9
Balance at end of period	30.7	29.7	30.7	29.7
Retail Loans [Member] | Acquired Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	0.2	0.1	0.2	0.2
Provision for loan losses		0.1
Balance at end of period	$ 0.2	$ 0.2	$ 0.2	$ 0.2

Loans - Summary of Allowance fo

Loans - Summary of Allowance for Loan Losses by Loan Portfolio Segment and Impairment Methodology (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Jun. 27, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, Portfolio	$ 32,199.3		$ 32,575.3
Total Allowance	238	$ 236.8	234.4	$ 233.4	$ 231.6	$ 229.3
Originated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment	29,900.5		29,877.9
Individually evaluated for impairment	210.3		233.3
Loans, Portfolio	30,110.8		30,111.2
Originated Loans Collectively Evaluated for Impairment, Allowance	223.6		223.8
Originated Loans Individually Evaluated for Impairment, Allowance	10.3		7
Financial Asset Acquired with Credit Deterioration [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, Portfolio	394.6		498.5
Total Allowance	2.3		3
Financial Asset Acquired and No Credit Deterioration [Member] | Purchased Performing Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment	1,689.1		1,963.7
Individually evaluated for impairment	4.8		1.9
Originated Loans Collectively Evaluated for Impairment, Allowance	1.7		0.6
Originated Loans Individually Evaluated for Impairment, Allowance	0.1
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, Portfolio	23,373.4		23,705.2
Total Allowance	207.1	205.8	204.5	203.5	201.9	204.9
Commercial [Member] | Originated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment	21,441.6		21,423.8
Individually evaluated for impairment	123.1		141.2
Loans, Portfolio	21,564.7		21,565
Originated Loans Collectively Evaluated for Impairment, Allowance	195.2		196.5
Originated Loans Individually Evaluated for Impairment, Allowance	8		4.6
Total Allowance	203.2	202	201.1	199.5	198.3	198.8
Commercial [Member] | Financial Asset Acquired with Credit Deterioration [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, Portfolio	284.3		370.4
Total Allowance	2.2		2.8
Commercial [Member] | Financial Asset Acquired and No Credit Deterioration [Member] | Purchased Performing Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment	1,521.2		1,769.8
Individually evaluated for impairment	3.2
Originated Loans Collectively Evaluated for Impairment, Allowance	1.7		0.6
Retail Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, Portfolio	8,825.9		8,870.1
Total Allowance	30.9	31	29.9	29.9	29.7	24.4
Retail Loans [Member] | Originated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment	8,458.9		8,454.1
Individually evaluated for impairment	87.2		92.1
Loans, Portfolio	8,546.1		8,546.2
Originated Loans Collectively Evaluated for Impairment, Allowance	28.4		27.3
Originated Loans Individually Evaluated for Impairment, Allowance	2.3		2.4
Total Allowance	30.7	$ 30.8	29.7	$ 29.7	$ 29.6	$ 24.2
Retail Loans [Member] | Financial Asset Acquired with Credit Deterioration [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, Portfolio	110.3		128.1
Total Allowance	0.1		0.2
Retail Loans [Member] | Financial Asset Acquired and No Credit Deterioration [Member] | Purchased Performing Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment	167.9		193.9
Individually evaluated for impairment	1.6		$ 1.9
Originated Loans Individually Evaluated for Impairment, Allowance	$ 0.1

Loans - Summarized Recorded Inv

Loans - Summarized Recorded Investments, by Class of Loan, in Originated Non-Performing Loans (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Recorded investments, non-performing loans	$ 158.1	$ 148.7
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Recorded investments, non-performing loans	111.4	100.6
Retail Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Recorded investments, non-performing loans	46.7	48.1
Commercial Real Estate Loan [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Recorded investments, non-performing loans	17.2	23.7
Commercial and Industrial [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Recorded investments, non-performing loans	44.9	32.6
Equipment Financing [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Recorded investments, non-performing loans	49.3	44.3
Residential Mortgage Loan [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Recorded investments, non-performing loans	32	32.7
Home Equity Loan [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Recorded investments, non-performing loans	14.6	$ 15.4
Other Consumer [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Recorded investments, non-performing loans	$ 0.1

Loans - Summarized Recorded I_2

Loans - Summarized Recorded Investments, by Class of Loan, in Originated Non-Performing Loans (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Government guarantees	$ 2.5	$ 3.1
Recorded investments, non-performing loans	158.1	148.7
Foreclosure [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Recorded investments, non-performing loans	$ 17.6	$ 15.2
Commercial Real Estate Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Guarantee rate	7.00%
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Guarantee rate	93.00%

Loans - Summary of Recorded Inv

Loans - Summary of Recorded Investments in TDRs by Class of Loan (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)Contracts	Sep. 30, 2017USD ($)Contracts	Sep. 30, 2018USD ($)Contracts	Sep. 30, 2017USD ($)Contracts
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	42	59	131	200
Pre-Modification Outstanding Recorded Investment	$ 44.9	$ 25.1	$ 109.7	$ 85.3
Post-Modification Outstanding Recorded Investment	$ 44.9	$ 25.1	$ 109.7	$ 85.3
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	19	23	59	90
Pre-Modification Outstanding Recorded Investment	$ 41.3	$ 19.8	$ 99.9	$ 70
Post-Modification Outstanding Recorded Investment	$ 41.3	$ 19.8	$ 99.9	$ 70
Retail Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	23	36	72	110
Pre-Modification Outstanding Recorded Investment	$ 3.6	$ 5.3	$ 9.8	$ 15.3
Post-Modification Outstanding Recorded Investment	$ 3.6	$ 5.3	$ 9.8	$ 15.3
Commercial Real Estate Loan [Member] | Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	4	1	9	8
Pre-Modification Outstanding Recorded Investment	$ 20.7	$ 0.1	$ 24.3	$ 5.3
Post-Modification Outstanding Recorded Investment	$ 20.7	$ 0.1	$ 24.3	$ 5.3
Commercial and Industrial [Member] | Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	10	9	34	28
Pre-Modification Outstanding Recorded Investment	$ 10.3	$ 11.6	$ 55.2	$ 39.6
Post-Modification Outstanding Recorded Investment	$ 10.3	$ 11.6	$ 55.2	$ 39.6
Equipment Financing [Member] | Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	5	13	16	54
Pre-Modification Outstanding Recorded Investment	$ 10.3	$ 8.1	$ 20.4	$ 25.1
Post-Modification Outstanding Recorded Investment	$ 10.3	$ 8.1	$ 20.4	$ 25.1
Residential Mortgage Loan [Member] | Retail Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	4	17	16	45
Pre-Modification Outstanding Recorded Investment	$ 1.4	$ 3.8	$ 4.9	$ 10.8
Post-Modification Outstanding Recorded Investment	$ 1.4	$ 3.8	$ 4.9	$ 10.8
Home Equity Loan [Member] | Retail Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	19	19	56	65
Pre-Modification Outstanding Recorded Investment	$ 2.2	$ 1.5	$ 4.9	$ 4.5
Post-Modification Outstanding Recorded Investment	$ 2.2	$ 1.5	$ 4.9	$ 4.5

Loans - Summary of Recorded I_2

Loans - Summary of Recorded Investments in TDRs by Class of Loan (Parenthetical) (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)Contracts	Sep. 30, 2017USD ($)Contracts	Sep. 30, 2018USD ($)Contracts	Sep. 30, 2017USD ($)Contracts
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	42	59	131	200
Recorded investment | $	$ 44.9	$ 25.1	$ 109.7	$ 85.3
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	19	23	59	90
Recorded investment | $	$ 41.3	$ 19.8	$ 99.9	$ 70
Commercial [Member] | Commercial Real Estate Loan [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	4	1	9	8
Recorded investment | $	$ 20.7	$ 0.1	$ 24.3	$ 5.3
Commercial [Member] | Commercial Real Estate Loan [Member] | Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	3	1	8	5
Recorded investment | $	$ 20.2	$ 0.1	$ 23.8	$ 1.4
Commercial [Member] | Commercial Real Estate Loan [Member] | Payment Deferral [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	1		1	2
Recorded investment | $	$ 0.5		$ 0.5	$ 2.2
Commercial [Member] | Commercial Real Estate Loan [Member] | Temporary Rate Reduction [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts				1
Recorded investment | $				$ 1.7
Commercial [Member] | Commercial and Industrial [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	10	9	34	28
Recorded investment | $	$ 10.3	$ 11.6	$ 55.2	$ 39.6
Commercial [Member] | Commercial and Industrial [Member] | Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	6	9	21	25
Recorded investment | $	$ 3.6	$ 11.6	$ 30.7	$ 29.9
Commercial [Member] | Commercial and Industrial [Member] | Payment Deferral [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	2		10	2
Recorded investment | $	$ 6.4		$ 23.8	$ 9.4
Commercial [Member] | Commercial and Industrial [Member] | Combination of Concessions [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	2		3	1
Recorded investment | $	$ 0.3		$ 0.7	$ 0.3
Commercial [Member] | Equipment Financing [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	5	13	16	54
Recorded investment | $	$ 10.3	$ 8.1	$ 20.4	$ 25.1
Commercial [Member] | Equipment Financing [Member] | Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	2		2
Recorded investment | $	$ 3.3		$ 3.3
Commercial [Member] | Equipment Financing [Member] | Payment Deferral [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	2	12	8	28
Recorded investment | $	$ 5.9	$ 8	$ 12.9	$ 17.4
Commercial [Member] | Equipment Financing [Member] | Combination of Concessions [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	1	1	6	26
Recorded investment | $	$ 1.1	$ 0.1	$ 4.2	$ 7.7
Retail Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	23	36	72	110
Recorded investment | $	$ 3.6	$ 5.3	$ 9.8	$ 15.3
Retail Loans [Member] | Residential Mortgage Loan [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	4	17	16	45
Recorded investment | $	$ 1.4	$ 3.8	$ 4.9	$ 10.8
Retail Loans [Member] | Residential Mortgage Loan [Member] | Payment Deferral [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	1	5	6	10
Recorded investment | $	$ 0.7	$ 1.7	$ 2.4	$ 3.4
Retail Loans [Member] | Residential Mortgage Loan [Member] | Combination of Concessions [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	1	3	5	7
Recorded investment | $	$ 0.1	$ 0.4	$ 1.6	$ 3.2
Retail Loans [Member] | Residential Mortgage Loan [Member] | Bankruptcy [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	2	9	5	28
Recorded investment | $	$ 0.6	$ 1.7	$ 0.9	$ 4.2
Retail Loans [Member] | Home Equity Loan [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	19	19	56	65
Recorded investment | $	$ 2.2	$ 1.5	$ 4.9	$ 4.5
Retail Loans [Member] | Home Equity Loan [Member] | Payment Deferral [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	6	4	10	10
Recorded investment | $	$ 1	$ 0.3	$ 1.3	$ 0.8
Retail Loans [Member] | Home Equity Loan [Member] | Combination of Concessions [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	2	5	9	14
Recorded investment | $	$ 0.1	$ 0.2	$ 0.8	$ 1.1
Retail Loans [Member] | Home Equity Loan [Member] | Bankruptcy [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	11	10	37	41
Recorded investment | $	$ 1.1	$ 1	$ 2.8	$ 2.6

Loans - Summary of Recorded I_3

Loans - Summary of Recorded Investments in TDRs by Class of Loan, Subsequently Defaulted (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)Contracts	Sep. 30, 2017USD ($)Contracts	Sep. 30, 2018USD ($)Contracts	Sep. 30, 2017USD ($)Contracts
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	18	16	38	39
Recorded Investment | $	$ 6.7	$ 6	$ 18	$ 12.3
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	5	9	22	19
Recorded Investment | $	$ 5.2	$ 4.3	$ 16.1	$ 8.3
Commercial [Member] | Commercial Real Estate Loan [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts			2
Recorded Investment | $			$ 0.8
Commercial [Member] | Commercial and Industrial [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	4	1	13	4
Recorded Investment | $	$ 5	$ 0.1	$ 8.5	$ 1.5
Commercial [Member] | Equipment Financing [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	1	8	7	15
Recorded Investment | $	$ 0.2	$ 4.2	$ 6.8	$ 6.8
Retail Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	13	7	16	20
Recorded Investment | $	$ 1.5	$ 1.7	$ 1.9	$ 4
Retail Loans [Member] | Residential Mortgage Loan [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	5	2	7	9
Recorded Investment | $	$ 0.9	$ 0.9	$ 1.3	$ 2.7
Retail Loans [Member] | Home Equity Loan [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contracts	8	5	9	11
Recorded Investment | $	$ 0.6	$ 0.8	$ 0.6	$ 1.3

Loans - Summary of Individually

Loans - Summary of Individually-Evaluated Impaired Loans by Class of Loan (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
With no related allowance for loan losses, Unpaid Principal Balance	$ 179.3	$ 194.1
With no related allowance for loan losses, Recorded Investment	161	175.6
With a related allowance for loan losses, Unpaid Principal Balance	60.9	63.3
With a related allowance for loan losses, Recorded Investment	54.1	59.6
Unpaid Principal Balance	240.2	257.4
Recorded Investment	215.1	235.2
Related Allowance for Loan Losses	10.4	7
Commercial [Member]
Financing Receivable, Impaired [Line Items]
With a related allowance for loan losses, Recorded Investment	42
Unpaid Principal Balance	140.6	152.7
Recorded Investment	126.3	141.2
Related Allowance for Loan Losses	8	4.6
Commercial [Member] | Commercial Real Estate Loan [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance for loan losses, Unpaid Principal Balance	31.2	37.7
With no related allowance for loan losses, Recorded Investment	29.2	36.3
With a related allowance for loan losses, Unpaid Principal Balance	10.3	11.7
With a related allowance for loan losses, Recorded Investment	7.4	9.9
Unpaid Principal Balance	41.5	49.4
Recorded Investment	36.6	46.2
Related Allowance for Loan Losses	0.8	0.9
Commercial [Member] | Commercial and Industrial [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance for loan losses, Unpaid Principal Balance	29.3	27.9
With no related allowance for loan losses, Recorded Investment	27.1	25.5
With a related allowance for loan losses, Unpaid Principal Balance	25.9	26.9
With a related allowance for loan losses, Recorded Investment	22.7	26
Unpaid Principal Balance	55.2	54.8
Recorded Investment	49.8	51.5
Related Allowance for Loan Losses	4.2	2.6
Commercial [Member] | Equipment Financing [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance for loan losses, Unpaid Principal Balance	31.3	36.9
With no related allowance for loan losses, Recorded Investment	28	32.8
With a related allowance for loan losses, Unpaid Principal Balance	12.6	11.6
With a related allowance for loan losses, Recorded Investment	11.9	10.7
Unpaid Principal Balance	43.9	48.5
Recorded Investment	39.9	43.5
Related Allowance for Loan Losses	3	1.1
Retail Loans [Member]
Financing Receivable, Impaired [Line Items]
With a related allowance for loan losses, Recorded Investment	12.1
Unpaid Principal Balance	99.6	104.7
Recorded Investment	88.8	94
Related Allowance for Loan Losses	2.4	2.4
Retail Loans [Member] | Residential Mortgage Loan [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance for loan losses, Unpaid Principal Balance	64	67.6
With no related allowance for loan losses, Recorded Investment	56.8	60.8
With a related allowance for loan losses, Unpaid Principal Balance	10.5	11.4
With a related allowance for loan losses, Recorded Investment	10.5	11.4
Unpaid Principal Balance	74.5	79
Recorded Investment	67.3	72.2
Related Allowance for Loan Losses	1.8	1.7
Retail Loans [Member] | Home Equity Loan [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance for loan losses, Unpaid Principal Balance	23.5	24
With no related allowance for loan losses, Recorded Investment	19.9	20.2
With a related allowance for loan losses, Unpaid Principal Balance	1.6	1.7
With a related allowance for loan losses, Recorded Investment	1.6	1.6
Unpaid Principal Balance	25.1	25.7
Recorded Investment	21.5	21.8
Related Allowance for Loan Losses	$ 0.6	$ 0.7

Loans - Schedule of Impaired Fi

Loans - Schedule of Impaired Financing Receivable (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	$ 213.9	$ 262.2	$ 221.6	$ 259
Interest Income Recognized	1.3	1.7	4.1	4.5
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	124.6	168	131.1	166
Interest Income Recognized	0.7	1.2	2.4	2.9
Commercial [Member] | Commercial Real Estate Loan [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	36.2	61.8	40.7	56.9
Interest Income Recognized	0.4	0.3	0.9	0.9
Commercial [Member] | Commercial and Industrial [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	52.4	59.7	50.6	66.1
Interest Income Recognized	0.2	0.7	1.3	1.6
Commercial [Member] | Equipment Financing [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	36	46.5	39.8	43
Interest Income Recognized	0.1	0.2	0.2	0.4
Retail Loans [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	89.3	94.2	90.5	93
Interest Income Recognized	0.6	0.5	1.7	1.6
Retail Loans [Member] | Residential Mortgage Loan [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	68.6	72.4	69.5	71.9
Interest Income Recognized	0.4	0.4	1.3	1.3
Retail Loans [Member] | Home Equity Loan [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	20.7	21.8	21	21.1
Interest Income Recognized	$ 0.2	$ 0.1	$ 0.4	$ 0.3

Loans - Summary of Aging Inform

Loans - Summary of Aging Information by Class of Loan (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	$ 32,199.3	$ 32,575.3
Originated [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	29,895	29,898.8
Total Past Due	215.8	212.4
Total loans	30,110.8	30,111.2
Originated [Member] | 30-89 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	138.3	149.5
Originated [Member] | 90 Days or More [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	77.5	62.9
Retail Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	8,825.9	8,870.1
Retail Loans [Member] | Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	46.8	49.2
Retail Loans [Member] | Home Equity Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	1,867.2	2,015.2
Retail Loans [Member] | Residential Mortgage Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	6,911.9	6,805.7
Retail Loans [Member] | Originated [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	8,479.9	8,477.8
Total Past Due	66.2	68.4
Total loans	8,546.1	8,546.2
Retail Loans [Member] | Originated [Member] | Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	44	45.3
Total Past Due	0.2	0.3
Total loans	44.2	45.6
Retail Loans [Member] | Originated [Member] | Home Equity Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,810.6	1,945.2
Total Past Due	13	14.8
Total loans	1,823.6	1,960
Retail Loans [Member] | Originated [Member] | Residential Mortgage Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	6,625.3	6,487.3
Total Past Due	53	53.3
Total loans	6,678.3	6,540.6
Retail Loans [Member] | Originated [Member] | 30-89 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	38.4	40.5
Retail Loans [Member] | Originated [Member] | 30-89 Days [Member] | Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0.1	0.3
Retail Loans [Member] | Originated [Member] | 30-89 Days [Member] | Home Equity Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6.4	7.4
Retail Loans [Member] | Originated [Member] | 30-89 Days [Member] | Residential Mortgage Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	31.9	32.8
Retail Loans [Member] | Originated [Member] | 90 Days or More [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	27.8	27.9
Retail Loans [Member] | Originated [Member] | 90 Days or More [Member] | Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0.1
Retail Loans [Member] | Originated [Member] | 90 Days or More [Member] | Home Equity Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6.6	7.4
Retail Loans [Member] | Originated [Member] | 90 Days or More [Member] | Residential Mortgage Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	21.1	20.5
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	23,373.4	23,705.2
Commercial [Member] | Commercial Real Estate Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	10,595.5	11,068.7
Commercial [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	8,568.6	8,731.1
Commercial [Member] | Equipment Financing [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	4,209.3	3,905.4
Commercial [Member] | Originated [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	21,415.1	21,421
Total Past Due	149.6	144
Total loans	21,564.7	21,565
Commercial [Member] | Originated [Member] | Commercial Real Estate Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	9,728.2	10,102.3
Total Past Due	22.7	24.3
Total loans	9,750.9	10,126.6
Commercial [Member] | Originated [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	8,044.5	8,099
Total Past Due	34.4	30.9
Total loans	8,078.9	8,129.9
Commercial [Member] | Originated [Member] | Equipment Financing [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	3,642.4	3,219.7
Total Past Due	92.5	88.8
Total loans	3,734.9	3,308.5
Commercial [Member] | Originated [Member] | 30-89 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	99.9	109
Commercial [Member] | Originated [Member] | 30-89 Days [Member] | Commercial Real Estate Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	10.1	11
Commercial [Member] | Originated [Member] | 30-89 Days [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	14.2	14.9
Commercial [Member] | Originated [Member] | 30-89 Days [Member] | Equipment Financing [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	75.6	83.1
Commercial [Member] | Originated [Member] | 90 Days or More [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	49.7	35
Commercial [Member] | Originated [Member] | 90 Days or More [Member] | Commercial Real Estate Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	12.6	13.3
Commercial [Member] | Originated [Member] | 90 Days or More [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	20.2	16
Commercial [Member] | Originated [Member] | 90 Days or More [Member] | Equipment Financing [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 16.9	$ 5.7

Loans - Summary of Credit Quali

Loans - Summary of Credit Quality Indicators by Class of Loan (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	$ 32,199.3	$ 32,575.3
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	23,373.4	23,705.2
Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	8,825.9	8,870.1
Originated [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	21,564.7	21,565
Originated [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	20,496.5	20,519.9
Originated [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	372.8	375.2
Originated [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	694.2	668
Originated [Member] | Commercial [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	1.2	1.9
Originated [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	8,546.1	8,546.2
Originated [Member] | Retail Loans [Member] | Low Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	4,233.3	4,245.9
Originated [Member] | Retail Loans [Member] | Moderate Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	3,425.2	3,385.9
Originated [Member] | Retail Loans [Member] | High Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	887.6	914.4
Acquired Loans [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	1,808.7	2,140.2
Acquired Loans [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	1,673.8	2,008.9
Acquired Loans [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	38.7	30
Acquired Loans [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	96.2	101.3
Acquired Loans [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	279.8	323.9
Acquired Loans [Member] | Retail Loans [Member] | Low Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	136.6	148
Acquired Loans [Member] | Retail Loans [Member] | Moderate Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	52.3	65.7
Acquired Loans [Member] | Retail Loans [Member] | High Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	90.9	110.2
Commercial Real Estate Loan [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	10,595.5	11,068.7
Commercial Real Estate Loan [Member] | Originated [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	9,750.9	10,126.6
Commercial Real Estate Loan [Member] | Originated [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	9,500	9,859.3
Commercial Real Estate Loan [Member] | Originated [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	134.2	159.4
Commercial Real Estate Loan [Member] | Originated [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	115.9	107
Commercial Real Estate Loan [Member] | Originated [Member] | Commercial [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	0.8	0.9
Commercial Real Estate Loan [Member] | Acquired Loans [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	844.6	942.1
Commercial Real Estate Loan [Member] | Acquired Loans [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	779.9	892
Commercial Real Estate Loan [Member] | Acquired Loans [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	26.2	14.8
Commercial Real Estate Loan [Member] | Acquired Loans [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	38.5	35.3
Commercial and Industrial [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	8,568.6	8,731.1
Commercial and Industrial [Member] | Originated [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	8,078.9	8,129.9
Commercial and Industrial [Member] | Originated [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	7,644.2	7,760.7
Commercial and Industrial [Member] | Originated [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	170.1	124
Commercial and Industrial [Member] | Originated [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	264.2	244.2
Commercial and Industrial [Member] | Originated [Member] | Commercial [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	0.4	1
Commercial and Industrial [Member] | Acquired Loans [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	489.7	601.2
Commercial and Industrial [Member] | Acquired Loans [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	425.8	520
Commercial and Industrial [Member] | Acquired Loans [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	9.3	15.2
Commercial and Industrial [Member] | Acquired Loans [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	54.6	66
Equipment Financing [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	4,209.3	3,905.4
Equipment Financing [Member] | Originated [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	3,734.9	3,308.5
Equipment Financing [Member] | Originated [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	3,352.3	2,899.9
Equipment Financing [Member] | Originated [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	68.5	91.8
Equipment Financing [Member] | Originated [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	314.1	316.8
Equipment Financing [Member] | Acquired Loans [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	474.4	596.9
Equipment Financing [Member] | Acquired Loans [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	468.1	596.9
Equipment Financing [Member] | Acquired Loans [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	3.2
Equipment Financing [Member] | Acquired Loans [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	3.1
Residential Mortgage Loan [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	6,911.9	6,805.7
Residential Mortgage Loan [Member] | Originated [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	6,678.3	6,540.6
Residential Mortgage Loan [Member] | Originated [Member] | Retail Loans [Member] | Low Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	3,343.1	3,292.1
Residential Mortgage Loan [Member] | Originated [Member] | Retail Loans [Member] | Moderate Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	2,829	2,738.8
Residential Mortgage Loan [Member] | Originated [Member] | Retail Loans [Member] | High Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	506.2	509.7
Residential Mortgage Loan [Member] | Acquired Loans [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	233.6	265.1
Residential Mortgage Loan [Member] | Acquired Loans [Member] | Retail Loans [Member] | Low Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	136.6	148
Residential Mortgage Loan [Member] | Acquired Loans [Member] | Retail Loans [Member] | Moderate Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	52.3	65.7
Residential Mortgage Loan [Member] | Acquired Loans [Member] | Retail Loans [Member] | High Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	44.7	51.4
Home Equity Loan [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	1,867.2	2,015.2
Home Equity Loan [Member] | Originated [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	1,823.6	1,960
Home Equity Loan [Member] | Originated [Member] | Retail Loans [Member] | Low Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	862	925.6
Home Equity Loan [Member] | Originated [Member] | Retail Loans [Member] | Moderate Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	589.6	640
Home Equity Loan [Member] | Originated [Member] | Retail Loans [Member] | High Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	372	394.4
Home Equity Loan [Member] | Acquired Loans [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	43.6	55.2
Home Equity Loan [Member] | Acquired Loans [Member] | Retail Loans [Member] | High Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	43.6	55.2
Other Consumer [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	46.8	49.2
Other Consumer [Member] | Originated [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	44.2	45.6
Other Consumer [Member] | Originated [Member] | Retail Loans [Member] | Low Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	28.2	28.2
Other Consumer [Member] | Originated [Member] | Retail Loans [Member] | Moderate Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	6.6	7.1
Other Consumer [Member] | Originated [Member] | Retail Loans [Member] | High Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	9.4	10.3
Other Consumer [Member] | Acquired Loans [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	2.6	3.6
Other Consumer [Member] | Acquired Loans [Member] | Retail Loans [Member] | High Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	$ 2.6	$ 3.6

Loans - Summarized Activity in

Loans - Summarized Activity in Accretable Yield for PCI Loan Portfolio (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Receivables [Abstract]
Balance at beginning of period	$ 190.2	$ 233.4	$ 219.7	$ 255.4
Acquisitions				13.1
Accretion	(5.6)	(7.1)	(17.9)	(22.5)
Reclassification from nonaccretable difference for loans with improved cash flows	0	0	0	0
Other changes in expected cash flows			(17.2)	(19.7)
Balance at end of period	$ 184.6	$ 226.3	$ 184.6	$ 226.3

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Stockholders Equity [Line Items]
Preferred stock, shares authorized	50,000,000	50,000,000
Preferred stock par value, per share	$ 0.01	$ 0.01
Common stock, shares authorized	1,950,000,000	1,950,000,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares issued	437,700,000	435,600,000
Preferred stock, shares outstanding	10,000,000	10,000,000
Number of common stock purchased	89,000,000	89,000,000
People's United Financial, Inc. [Member]
Schedule Of Stockholders Equity [Line Items]
Preferred stock, shares authorized	50,000,000	50,000,000
Preferred stock par value, per share	$ 0.01	$ 0.01
Common stock, shares authorized	1,950,000,000	1,950,000,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares issued	437,700,000	435,600,000
Preferred stock, shares outstanding	10,000,000	10,000,000
Repurchases Authorized by Board of Directors [Member]
Schedule Of Stockholders Equity [Line Items]
Number of common stock purchased	86,400,000	86,400,000
2007 Recognition and Retention Plan [Member]
Schedule Of Stockholders Equity [Line Items]
Number of common stock purchased	2,600,000	2,600,000
2014 Long-Term Incentive Plan [Member] | 2007 Recognition and Retention Plan [Member]
Schedule Of Stockholders Equity [Line Items]
Additional awards under RRP	0	0

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			$ 5,819.9	$ 5,141.9
Amounts reclassified from AOCL	$ 2.3	$ 1.3	6.8	13.9
Current period other comprehensive income	(10.1)	4.9	(51.8)	25.3
Transition adjustments related to adoption of new accounting standards	0.6		0.6
Ending Balance	5,958.9	5,746.1	5,958.9	5,746.1
Pension and Other Postretirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(144.1)	(145.6)
Amounts reclassified from AOCL			4.5	2.8
Current period other comprehensive income			4.5	2.8
Transition adjustments related to adoption of new accounting standards	(30)		(30)
Ending Balance	(169.6)	(142.8)	(169.6)	(142.8)
Net Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(21.6)	(32.3)
Other comprehensive income (loss) before reclassifications			(56.2)	11.1
Amounts reclassified from AOCL			(0.1)	9.9
Current period other comprehensive income			(56.3)	21
Transition adjustments related to adoption of new accounting standards	(3.9)		(3.9)
Ending Balance	(81.8)	(11.3)	(81.8)	(11.3)
Net Unrealized Gain (Losses) On Debt Securities Transferred To Held To Maturity [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(15.1)	(17.4)
Amounts reclassified from AOCL			2.3	1.6
Current period other comprehensive income			2.3	1.6
Transition adjustments related to adoption of new accounting standards	(3.2)		(3.2)
Ending Balance	(16)	(15.8)	(16)	(15.8)
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(0.9)	0.3
Other comprehensive income (loss) before reclassifications			(2.4)	0.3
Amounts reclassified from AOCL			0.1	(0.4)
Current period other comprehensive income			(2.3)	(0.1)
Transition adjustments related to adoption of new accounting standards	(0.2)		(0.2)
Ending Balance	(3.4)	0.2	(3.4)	0.2
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(181.7)	(195)
Other comprehensive income (loss) before reclassifications			(58.6)	11.4
Amounts reclassified from AOCL			6.8	13.9
Current period other comprehensive income			(51.8)	25.3
Transition adjustments related to adoption of new accounting standards	(37.3)		(37.3)
Ending Balance	$ (270.8)	$ (169.7)	$ (270.8)	$ (169.7)

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Amounts Reclassified from AOCL (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense	$ (149.2)	$ (129.8)	$ (427.5)	$ (334.7)
Income tax expense	32.2	39	92.4	103.8
Net income	(117)	(90.8)	(335.1)	(230.9)
Interest expense - notes and debentures	8.5	7.5	24.7	22.3
Total reclassifications for the period	(2.3)	(1.3)	(6.8)	(13.9)
Pension and Other Postretirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial loss	(2)	(1.7)	(6.2)	(5.1)
Prior service credit		0.2	0.2	0.6
Income before income tax expense	(2)	(1.5)	(6)	(4.5)
Income tax expense	0.5	0.6	1.5	1.7
Net income	(1.5)	(0.9)	(4.5)	(2.8)
Total reclassifications for the period			(4.5)	(2.8)
Net Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense			0.1	(15.6)
Income tax expense				5.7
Net income			0.1	(9.9)
Total reclassifications for the period			0.1	(9.9)
Net Unrealized Gain (Losses) On Debt Securities Transferred To Held To Maturity [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense	(0.9)	(0.9)	(3)	(2.6)
Income tax expense	0.2	0.3	0.7	1
Net income	(0.7)	(0.6)	(2.3)	(1.6)
Total reclassifications for the period			(2.3)	(1.6)
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense	(0.2)	0.4	(0.2)	0.7
Income tax expense	0.1	(0.2)	0.1	(0.3)
Net income	(0.1)	0.2	(0.1)	0.4
Total reclassifications for the period			(0.1)	0.4
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member] | Interest Rate Swaps [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest expense - notes and debentures	(0.3)	0.3	(0.3)	0.6
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member] | Interest Rate Locks [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest expense - notes and debentures	$ 0.1	$ 0.1	$ 0.1	$ 0.1

Earnings Per Common Share - Bas

Earnings Per Common Share - Basic and Diluted Earnings Per Common Share, Reflecting Application of Two-Class Method (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income available to common shareholders	$ 113.5	$ 87.3	$ 324.6	$ 220.4
Dividends paid on and undistributed earnings allocated to participating securities		(0.1)	(0.2)	(0.4)
Earnings attributable to common shareholders	$ 113.5	$ 87.2	$ 324.4	$ 220
Weighted average common shares outstanding for basic EPS	341.4	336.9	340.6	327.6
Effect of dilutive equity-based awards	3.6	1.9	3.9	2
Weighted average common shares and common-equivalent shares for diluted EPS	345	338.8	344.5	329.6
EPS
Basic	$ 0.33	$ 0.26	$ 0.95	$ 0.67
Diluted	$ 0.33	$ 0.26	$ 0.94	$ 0.67

Earnings Per Common Share - Add

Earnings Per Common Share - Additional Information (Detail) - shares shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Anti-dilutive equity-based awards excluded from calculation of diluted EPS	6.8	10.2	6.8	10.1

Goodwill and Other Acquisitio_3

Goodwill and Other Acquisition-Related Intangible Assets - Schedule of Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Goodwill [Line Items]
Beginning balance	$ 2,411.4	$ 1,992.7
Ending balance	2,435.2	2,411.4
Vend Lease [Member]
Goodwill [Line Items]
Acquisition	23.8
Suffolk Bancorp [Member]
Goodwill [Line Items]
Acquisition		270.3
LEAF Commercial Capital, Inc. [Member]
Goodwill [Line Items]
Acquisition		148.4
Commercial Banking Loan [Member]
Goodwill [Line Items]
Beginning balance	1,600.3	1,222.1
Ending balance	1,624.1	1,600.3
Commercial Banking Loan [Member] | Vend Lease [Member]
Goodwill [Line Items]
Acquisition	23.8
Commercial Banking Loan [Member] | Suffolk Bancorp [Member]
Goodwill [Line Items]
Acquisition		229.8
Commercial Banking Loan [Member] | LEAF Commercial Capital, Inc. [Member]
Goodwill [Line Items]
Acquisition		148.4
Retail Banking [Member]
Goodwill [Line Items]
Beginning balance	720.1	679.6
Ending balance	720.1	720.1
Retail Banking [Member] | Suffolk Bancorp [Member]
Goodwill [Line Items]
Acquisition		40.5
Wealth Management [Member]
Goodwill [Line Items]
Beginning balance	91	91
Ending balance	$ 91	$ 91

Goodwill and Other Acquisitio_4

Goodwill and Other Acquisition-Related Intangible Assets - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Goodwill And Other Intangible Assets [Line Items]
Tax deductible goodwill amount	$ 91,500,000		$ 91,500,000		$ 72,300,000
Other acquisition-related intangible assets	133,700,000		133,700,000		$ 148,600,000
Amortization of other acquisition-related intangible assets	4,900,000	$ 7,900,000	14,900,000	$ 22,100,000
Amortization expense attributable to other acquisition-related intangible assets, 2018	19,900,000		19,900,000
Amortization expense attributable to other acquisition-related intangible assets, 2019	18,400,000		18,400,000
Amortization expense attributable to other acquisition-related intangible assets, 2020	16,800,000		16,800,000
Amortization expense attributable to other acquisition-related intangible assets, 2021	15,000,000		15,000,000
Amortization expense attributable to other acquisition-related intangible assets, 2022	13,500,000		13,500,000
Amortization expense attributable to other acquisition-related intangible assets, 2023	8,500,000		8,500,000
Impairment losses relating to goodwill or other acquisition-related intangible assets			0	$ 0
Mutual Fund Management Contracts [Member]
Goodwill And Other Intangible Assets [Line Items]
Mutual fund management contracts (not amortized)	16,500,000		16,500,000
Trade Name [Member]
Goodwill And Other Intangible Assets [Line Items]
Other acquisition-related intangible assets	60,900,000		60,900,000
Client Relationship [Member]
Goodwill And Other Intangible Assets [Line Items]
Other acquisition-related intangible assets	20,500,000		20,500,000
Customer Relationships [Member]
Goodwill And Other Intangible Assets [Line Items]
Other acquisition-related intangible assets	12,100,000		12,100,000
Core Deposit Intangibles [Member]
Goodwill And Other Intangible Assets [Line Items]
Other acquisition-related intangible assets	18,100,000		18,100,000
Insurance Relationships [Member]
Goodwill And Other Intangible Assets [Line Items]
Other acquisition-related intangible assets	4,700,000		4,700,000
Favorable Lease Agreement [Member]
Goodwill And Other Intangible Assets [Line Items]
Other acquisition-related intangible assets	500,000		500,000
Noncompete Agreements [Member]
Goodwill And Other Intangible Assets [Line Items]
Other acquisition-related intangible assets	$ 400,000		$ 400,000

Employee Benefit Plans - People

Employee Benefit Plans - People's United Employee Pension and Other Postretirement Plans - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2018USD ($)
Defined Benefit Plan Disclosure [Line Items]
Average compensation term on which pension plan benefits are based, in years	5 years
Average compensation term within last ten years on which pension plan benefits are based, in years	5 years
Term considered when determining employee pension benefits, in years	10 years
Minimum age requirement to participate in pension plan, years	18 years
Minimum service hours per year requirement to participate in pension plan	1000 hours
People's Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of covered employee's eligible compensation	3.00%
Expected employer contributions	$ 40,000,000
Chittenden Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions	$ 10,000,000

Employee Benefit Plans - Compon

Employee Benefit Plans - Components of Net Periodic Benefit (Income) Expense and Other Amounts (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Other Postretirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 0.2	$ 0.2	$ 0.6	$ 0.6
Recognized net actuarial loss		0.1	0.2	0.2
Net periodic benefit (income) expense	0.2	0.3	0.8	0.8
Net actuarial loss			(0.2)	(0.2)
Total pre-tax changes recognized in other comprehensive income (loss)			(0.2)	(0.2)
Total recognized in net periodic benefit (income) expense and other comprehensive income (loss)			0.6	0.6
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	4.9	4.9	14.6	14.3
Expected return on plan assets	(10.9)	(9.6)	(32.7)	(28.3)
Recognized net actuarial loss	2	1.6	6	4.9
Recognized prior service credit		(0.2)	(0.2)	(0.6)
Settlements	(0.5)	1.1	0.4	2.6
Net periodic benefit (income) expense	$ (4.5)	$ (2.2)	(11.9)	(7.1)
Net actuarial loss			(6)	(4.9)
Prior service credit			0.2	0.6
Total pre-tax changes recognized in other comprehensive income (loss)			(5.8)	(4.3)
Total recognized in net periodic benefit (income) expense and other comprehensive income (loss)			$ (17.7)	$ (11.4)

Employee Benefit Plans - Employ

Employee Benefit Plans - Employee Stock Ownership Plan - Additional Information (Detail) - USD ($) $ in Millions	Apr. 30, 2007	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
ESOP, shares to be purchased	10,453,575
ESOP loan	$ 216.8	$ 180.8
Loan repayments expected annual through 2036		18.8
Cash dividends paid on unallocated ESOP shares		$ 3.5
Minimum age requirement to participate in ESOP		18 years
Minimum work experience within 12 months of hire needed to participate in ESOP		1000 hours
ESOP common stock allocated		4,094,316
Unallocated common stock of Employee Stock Ownership Plan, shares		6,359,259		6,600,000
Fair value of deferred ESOP shares		$ 108.9
ESOP compensation expense		$ 4.8	$ 4.8

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value transfers between level 1 and level 2	$ 0	$ 0
GSE Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maturity period of available-for-sale residential mortgage-backed securities portfolio	10 years		15 years
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount to impaired loans	10.00%

Fair Value Measurements - Asset

Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading debt securities	$ 8.3	$ 8.2
Other assets	1,013.7	952.7
Fair Values, Assets	50.3	77.9
Fair Values, Liabilities	211.6	88.3
Fair value of total assets measured at fair value on a recurring basis	3,422.1	3,261.4
Fair value of total liabilities measured at fair value on a recurring basis	211.6	88.3
Exchange Traded Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	39.1	36.5
Equity Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	3.1	3.5
Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0.3	1.3
US Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading debt securities	8.3	8.2
U.S. Treasury and Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value	652.1	668.8
GSE Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value	2,660	2,456.5
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value	8.9	8.7
Interest Rate Swaps [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Values, Assets	45.9	74.8
Fair Values, Liabilities	207.4	84.9
Interest Rate Caps [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Values, Assets	3.6	2.8
Fair Values, Liabilities	3.6	2.8
Foreign Exchange Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Values, Assets	0.6	0.1
Fair Values, Liabilities	0.4	0.4
Forward Commitments to Sell Residential Mortgage Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Values, Assets	0.2	0.2
Interest Rate-Lock Commitments on Residential Mortgage Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Values, Liabilities	0.2	0.2
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of total assets measured at fair value on a recurring basis	711.5	725.7
Level 1 [Member] | Exchange Traded Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	39.1	36.5
Level 1 [Member] | Equity Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	3.1	3.5
Level 1 [Member] | US Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading debt securities	8.3	8.2
Level 1 [Member] | U.S. Treasury and Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value	652.1	668.8
Level 1 [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value	8.9	8.7
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of total assets measured at fair value on a recurring basis	2,710.6	2,535.7
Fair value of total liabilities measured at fair value on a recurring basis	211.6	88.3
Level 2 [Member] | Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0.3	1.3
Level 2 [Member] | GSE Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value	2,660	2,456.5
Level 2 [Member] | Interest Rate Swaps [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Values, Assets	45.9	74.8
Fair Values, Liabilities	207.4	84.9
Level 2 [Member] | Interest Rate Caps [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Values, Assets	3.6	2.8
Fair Values, Liabilities	3.6	2.8
Level 2 [Member] | Foreign Exchange Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Values, Assets	0.6	0.1
Fair Values, Liabilities	0.4	0.4
Level 2 [Member] | Forward Commitments to Sell Residential Mortgage Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Values, Assets	0.2	0.2
Level 2 [Member] | Interest Rate-Lock Commitments on Residential Mortgage Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Values, Liabilities	$ 0.2	$ 0.2

Fair Value Measurements - Ass_2

Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Parenthetical) (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Risk Participation Agreements [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Derivatives, Net	$ 100,000	$ 100,000

Fair Value Measurements - Ass_3

Fair Value Measurements - Assets Measured at Fair Value on Non-Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	$ 15.2	$ 16.6
Impaired loans	54.1	59.6
Fair value of total assets measured at fair value on a recurring basis	3,422.1	3,261.4
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of total assets measured at fair value on a recurring basis	2,710.6	2,535.7
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	15.2	16.6
Impaired loans	54.1	59.6
REO and repossessed assets	15.1	19.4
Fair value of total assets measured at fair value on a recurring basis	84.4	95.6
Fair Value, Measurements, Nonrecurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	15.2	16.6
Fair value of total assets measured at fair value on a recurring basis	15.2	16.6
Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	54.1	59.6
REO and repossessed assets	15.1	19.4
Fair value of total assets measured at fair value on a recurring basis	$ 69.2	$ 79

Fair Value Measurements - Ass_4

Fair Value Measurements - Assets Measured at Fair Value on Non-Recurring Basis (Parenthetical) (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	$ 54,100,000		$ 54,100,000		$ 59,600,000
Charge-offs to the allowance for loan losses related to loans	8,200,000	$ 7,000,000	20,100,000	$ 18,500,000
Repossessed assets	2,000,000		2,000,000		2,500,000
Impaired Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Charge-offs to the allowance for loan losses related to loans			7,700,000	4,100,000
REO and Repossessed Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Charge-offs to the allowance for loan losses related to loans			1,000,000	2,200,000
Nonoperating Income (Expense) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Write downs and net loss on sale of foreclosed/repossessed assets charged to non-interest expense total			100,000	(300,000)
Commercial Real Estate Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate owned	8,700,000		8,700,000		9,300,000
Residential Mortgage Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value adjustments			0	0
Real estate owned	4,400,000		4,400,000		7,600,000
Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	42,000,000		42,000,000
Charge-offs to the allowance for loan losses related to loans	8,000,000	5,600,000	18,200,000	9,600,000
Commercial [Member] | Commercial Real Estate Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	7,400,000		7,400,000		9,900,000
Retail Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	12,100,000		12,100,000
Charge-offs to the allowance for loan losses related to loans	200,000	$ 1,400,000	1,900,000	$ 8,900,000
Retail Loans [Member] | Residential Mortgage Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	$ 10,500,000		$ 10,500,000		$ 11,400,000

Fair Value Measurements - Carry

Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and due from banks	$ 410.5	$ 505.1
Short-term investments	127.5	377.5
Debt securities held-to-maturity	3,742.9	3,588.1
FHLB and FRB stock	312.4	312.3
Time deposits	6,035.9	5,454.3
FHLB advances	2,369.7	2,774.4
Federal funds purchased	735	820
Customer repurchase agreements	261.3	301.6
Other borrowings	26	207.8
Notes and debentures	885.6	901.6
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and due from banks	410.5	505.1
Short-term investments	127.5	377.5
Debt securities held-to-maturity	3,742.9	3,588.1
FHLB and FRB stock	312.4	312.3
Total loans, net	31,907.2	32,281.3
Time deposits	6,035.9	5,454.3
Other deposits	27,174.3	27,602
FHLB advances	2,369.7	2,774.4
Federal funds purchased	735	820
Customer repurchase agreements	261.3	301.6
Other borrowings	26	207.8
Notes and debentures	885.6	901.6
Estimated Fair Value Measurements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and due from banks	410.5	505.1
Short-term investments	127.5	377.5
Debt securities held-to-maturity	3,666.8	3,633.7
FHLB and FRB stock	312.4	312.3
Total loans, net	31,388.2	32,127.5
Time deposits	5,997.6	5,441.1
Other deposits	27,174.3	27,602
FHLB advances	2,370.2	2,775.3
Federal funds purchased	735	820
Customer repurchase agreements	261.3	301.6
Other borrowings	26	207.2
Notes and debentures	892.5	910.1
Estimated Fair Value Measurements [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and due from banks	410.5	505.1
Estimated Fair Value Measurements [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments	127.5	377.5
Debt securities held-to-maturity	3,665.3	3,632.2
FHLB and FRB stock	312.4	312.3
Total loans, net	6,582.7	6,632.2
Time deposits	5,997.6	5,441.1
Other deposits	27,174.3	27,602
FHLB advances	2,370.2	2,775.3
Federal funds purchased	735	820
Customer repurchase agreements	261.3	301.6
Other borrowings	26	207.2
Notes and debentures	892.5	910.1
Estimated Fair Value Measurements [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt securities held-to-maturity	1.5	1.5
Total loans, net	$ 24,805.5	$ 25,495.3

Fair Value Measurements - Car_2

Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Financial Instruments (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Impaired loans	$ 54.1	$ 59.6

Derivative Financial Instrume_3

Derivative Financial Instruments and Hedging Activities - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Derivative [Line Items]
Unrealized gain (loss) on derivatives	$ (3,100,000)	$ 400,000
Treasury Forward Interest Rate Locks ("T-Locks") [Member]
Derivative [Line Items]
Derivative instruments hedge description	To hedge the risk that the 10-year U.S. Treasury yield would rise
Unrealized gain (loss) on derivatives	$ 900,000
Period hedged items affected earnings, years	10 years
Treasury Forward Interest Rate Locks ("T-Locks") [Member] | Subordinated Notes [Member]
Derivative [Line Items]
Senior notes issuance	$ 500,000,000
Derivatives Designated as Hedging Instruments [Member]
Derivative [Line Items]
Unrealized gain (loss) on derivatives	(3,100,000)	400,000
Derivatives Designated as Hedging Instruments [Member] | Interest Rate Swaps [Member] | Cash Flow [Member]
Derivative [Line Items]
Unrealized gain (loss) on derivatives	$ (3,100,000)	$ 400,000
Derivatives Designated as Hedging Instruments [Member] | Interest Rate Swaps [Member] | Cash Flow [Member] | Loans [Member]
Derivative [Line Items]
LIBOR basis points	One-month LIBOR
Notional amount of derivatives	$ 210,000,000		$ 210,000,000
Derivatives Designated as Hedging Instruments [Member] | Interest Rate Swaps [Member] | Subordinated Notes [Member] | Fair Value [Member]
Derivative [Line Items]
LIBOR basis points	Three-month LIBOR
Notional amount of derivatives	$ 375,000,000
Subordinated notes	$ 400,000,000
Basis points	1.265%

Derivative Financial Instrume_4

Derivative Financial Instruments and Hedging Activities - Schedule of Notional Amounts and Fair Values of Derivatives Outstanding (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair Values, Liabilities	$ 211,600,000	$ 88,300,000
Fair value, Assets	50,300,000	77,900,000
Forward Commitments to Sell Residential Mortgage Loans [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair value, Assets	200,000	200,000
Interest Rate-Lock Commitments on Residential Mortgage Loans [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair Values, Liabilities	200,000	200,000
Derivatives Not Designated as Hedging Instruments [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair Values, Liabilities	211,600,000	88,300,000
Fair value, Assets	50,300,000	77,900,000
Derivatives Not Designated as Hedging Instruments [Member] | Forward Commitments to Sell Residential Mortgage Loans [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Amounts	18,900,000	16,400,000
Fair value, Assets	200,000	200,000
Derivatives Not Designated as Hedging Instruments [Member] | Interest Rate-Lock Commitments on Residential Mortgage Loans [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Amounts	24,000,000	18,300,000
Fair Values, Liabilities	200,000	200,000
Interest Rate Swaps [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Commercial Customers [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Amounts	6,576,200,000	5,769,100,000
Fair Values, Liabilities	199,100,000	61,200,000
Fair value, Assets	17,500,000	64,700,000
Interest Rate Swaps [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Institutional Counterparties [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Amounts	6,282,000,000	5,775,900,000
Fair Values, Liabilities	8,300,000	23,700,000
Fair value, Assets	28,400,000	10,100,000
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow [Member] | Loans [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Amounts	210,000,000	210,000,000
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Subordinated Notes [Member] | Fair Value [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Amounts	375,000,000	375,000,000
Interest Rate Caps [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Commercial Customers [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Amounts	313,000,000	649,200,000
Fair Values, Liabilities	3,600,000	2,800,000
Interest Rate Caps [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Institutional Counterparties [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Amounts	313,000,000	649,200,000
Fair value, Assets	3,600,000	2,800,000
Risk Participation Agreements [Member] | Derivatives Not Designated as Hedging Instruments [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Amounts	455,400,000	439,400,000
Foreign Exchange Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair Values, Liabilities	400,000	400,000
Fair value, Assets	600,000	100,000
Foreign Exchange Contracts [Member] | Derivatives Not Designated as Hedging Instruments [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Amounts	99,200,000	46,500,000
Fair Values, Liabilities	400,000	400,000
Fair value, Assets	$ 600,000	$ 100,000

Derivative Financial Instrume_5

Derivative Financial Instruments and Hedging Activities - Schedule of Notional Amounts and Fair Values of Derivatives Outstanding (Parenthetical) (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Risk Participation Agreements [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Maximum [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair Value of Derivatives, Net	$ 100,000	$ 100,000

Derivative Financial Instrume_6

Derivative Financial Instruments and Hedging Activities - Impact of Derivatives on Pre-Tax Income and Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	$ 6.3	$ 12.3
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	(3.1)	0.4
Derivatives Not Designated as Hedging Instruments [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	8.3	7.1
Derivatives Not Designated as Hedging Instruments [Member] | Forward Commitments to Sell Residential Mortgage Loans [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings		0.1
Derivatives Not Designated as Hedging Instruments [Member] | Interest Rate-Lock Commitments on Residential Mortgage Loans [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings		(0.1)
Derivatives Not Designated as Hedging Instruments [Member] | Interest Rate Swaps [Member] | Commercial Customers [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	(177.1)	44.2
Derivatives Not Designated as Hedging Instruments [Member] | Interest Rate Swaps [Member] | Institutional Counterparties [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	184.5	(38.4)
Derivatives Not Designated as Hedging Instruments [Member] | Interest Rate Caps [Member] | Commercial Customers [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	(0.7)	1.2
Derivatives Not Designated as Hedging Instruments [Member] | Interest Rate Caps [Member] | Institutional Counterparties [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	0.8	(0.7)
Derivatives Not Designated as Hedging Instruments [Member] | Foreign Exchange Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	0.6	0.3
Derivatives Not Designated as Hedging Instruments [Member] | Risk Participation Agreements [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	0.2	0.5
Derivatives Designated as Hedging Instruments [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	(2)	5.2
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	(3.1)	0.4
Derivatives Designated as Hedging Instruments [Member] | Cash Flow [Member] | Interest Rate-Lock Commitments on Residential Mortgage Loans [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	0.1	0.1
Derivatives Designated as Hedging Instruments [Member] | Interest Rate Swaps [Member] | Fair Value [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	(1.8)	4.5
Derivatives Designated as Hedging Instruments [Member] | Interest Rate Swaps [Member] | Cash Flow [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	(0.3)	0.6
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	$ (3.1)	$ 0.4

Balance Sheet Offsetting - Summ

Balance Sheet Offsetting - Summary of Gross Presentation, Financial Instruments that are Eligible for Offset in Consolidated Statement of Condition (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Offsetting Assets And Liabilities [Line Items]
Financial assets, Gross Amount Recognized	$ 50.3	$ 77.9
Financial liabilities, Gross Amount Recognized	211.6	88.3
Financial assets, Gross Amount Recognized	32.6	13
Financial assets, Gross Amount Offset		0
Financial assets, Net Amount Presented	32.6	13
Financial assets, Financial Instruments	(6.4)	(10.4)
Financial assets, Collateral	(25.5)	(2.4)
Financial assets, Net Amount	0.7	0.2
Financial liabilities, Gross Amount Recognized	8.7	24.1
Financial liabilities, Gross Amount Offset		0
Financial liabilities, Net Amount Presented	8.7	24.1
Financial liabilities, Financial Instruments	(6.4)	(10.4)
Financial liabilities, Collateral		(6)
Financial liabilities, Net Amount	2.3	7.7
Foreign Exchange Contracts [Member]
Offsetting Assets And Liabilities [Line Items]
Financial assets, Gross Amount Recognized	0.6	0.1
Financial assets, Net Amount Presented	0.6	0.1
Financial assets, Net Amount	0.6	0.1
Financial liabilities, Gross Amount Recognized	0.4	0.4
Financial liabilities, Net Amount Presented	0.4	0.4
Financial liabilities, Net Amount	0.4	0.4
Counterparty A [Member] | Interest Rate Swaps/Caps [Member]
Offsetting Assets And Liabilities [Line Items]
Financial assets, Gross Amount Recognized	5.1	2.6
Financial assets, Gross Amount Offset		0
Financial assets, Net Amount Presented	5.1	2.6
Financial assets, Financial Instruments	(1)	(2.5)
Financial assets, Collateral	(4.1)	0
Financial assets, Net Amount		0.1
Financial liabilities, Gross Amount Recognized	1	2.5
Financial liabilities, Gross Amount Offset		0
Financial liabilities, Net Amount Presented	1	2.5
Financial liabilities, Financial Instruments	(1)	(2.5)
Financial liabilities, Collateral		0
Financial liabilities, Net Amount		0
Counterparty B [Member] | Interest Rate Swaps/Caps [Member]
Offsetting Assets And Liabilities [Line Items]
Financial assets, Gross Amount Recognized	4.4	1.6
Financial assets, Net Amount Presented	4.4	1.6
Financial assets, Financial Instruments	(2.7)	(1.6)
Financial assets, Collateral	(1.7)
Financial liabilities, Gross Amount Recognized	2.7	5.6
Financial liabilities, Net Amount Presented	2.7	5.6
Financial liabilities, Financial Instruments	(2.7)	(1.6)
Financial liabilities, Collateral		(4)
Counterparty C [Member] | Interest Rate Swaps/Caps [Member]
Offsetting Assets And Liabilities [Line Items]
Financial assets, Gross Amount Recognized	10.3	2.6
Financial assets, Net Amount Presented	10.3	2.6
Financial assets, Financial Instruments	(1.2)	(2.6)
Financial assets, Collateral	(9.1)
Financial liabilities, Gross Amount Recognized	1.2	2.8
Financial liabilities, Net Amount Presented	1.2	2.8
Financial liabilities, Financial Instruments	(1.2)	(2.6)
Financial liabilities, Collateral		(0.2)
Counterparty D [Member] | Interest Rate Swaps/Caps [Member]
Offsetting Assets And Liabilities [Line Items]
Financial assets, Gross Amount Recognized	6.3	3.5
Financial assets, Net Amount Presented	6.3	3.5
Financial assets, Financial Instruments	(1.4)	(3.5)
Financial assets, Collateral	(4.9)
Financial liabilities, Gross Amount Recognized	1.4	4.7
Financial liabilities, Net Amount Presented	1.4	4.7
Financial liabilities, Financial Instruments	(1.4)	(3.5)
Financial liabilities, Collateral		(1.2)
Counterparty E [Member] | Interest Rate Swaps/Caps [Member]
Offsetting Assets And Liabilities [Line Items]
Financial liabilities, Gross Amount Recognized	1.9	7.3
Financial liabilities, Net Amount Presented	1.9	7.3
Financial liabilities, Net Amount	1.9	7.3
Other Counterparties [Member] | Interest Rate Swaps/Caps [Member]
Offsetting Assets And Liabilities [Line Items]
Financial assets, Gross Amount Recognized	5.9	2.6
Financial assets, Net Amount Presented	5.9	2.6
Financial assets, Financial Instruments	(0.1)	(0.2)
Financial assets, Collateral	(5.7)	(2.4)
Financial assets, Net Amount	0.1
Financial liabilities, Gross Amount Recognized	0.1	0.8
Financial liabilities, Net Amount Presented	0.1	0.8
Financial liabilities, Financial Instruments	$ (0.1)	(0.2)
Financial liabilities, Collateral		$ (0.6)

Balance Sheet Offsetting - Addi

Balance Sheet Offsetting - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
US Treasury [Member]
Offsettting Assets and Liabilities [Line Items]
Fair value of treasury securities	$ 463.9	$ 453.8
Mortgage-Backed Securities [Member]
Offsettting Assets and Liabilities [Line Items]
Fair value of treasury securities	$ 453.8	$ 461.9

Balance Sheet Offsetting - Su_2

Balance Sheet Offsetting - Summary of Collateral Swaps (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Offsettting Assets and Liabilities [Line Items]
Total repurchase agreements, Net amounts Presented in the Balance Sheet	$ 261.3	$ 301.6
Collateral Swaps [Member]
Offsettting Assets and Liabilities [Line Items]
Total resale agreements, Gross Amount Recognized	450	450
Total resale agreements, Gross Amount Offset in the Balance Sheet	(450)	(450)
Total resale agreements, Net amount Presented in the Balance Sheet		0
Total repurchase agreements, Gross Amount Recognized	450	450
Total repurchase agreements, Gross Amounts Offset in the Balance Sheet	$ (450)	(450)
Total repurchase agreements, Net amounts Presented in the Balance Sheet		$ 0

New Accounting Standards - Addi

New Accounting Standards - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 31, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Summary Of Significant Accounting Policies [Line Items]
Equity securities, at fair value				$ 8.9		$ 8.7
Future minimum lease payments due						353
Increase in risk-based capital ratios		0.11%
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of interest income on financial assets and financial liabilities				75.00%
Percentage of non-interest income on financial assets and financial liabilities				20.00%
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of interest income on financial assets and financial liabilities				80.00%
Percentage of non-interest income on financial assets and financial liabilities				25.00%
Term of contract with customers				1 year
Retained Earnings [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect transition adjustments		$ 37.9
Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect transition adjustments						0.6
Accounting Standards Update 2016-01 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect transition adjustments	$ (0.6)					(0.6)
Equity securities with an amortized cost						9.6
Equity securities, at fair value						$ 8.7
Accounting Standards Update 2017-07 [Member] | Net periodic pension and postretirement benefit income, reclassification [Member]
Summary Of Significant Accounting Policies [Line Items]
New accounting pronouncement, effect of change on operating results			$ 1.9		$ 6.3

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) shares in Millions, $ in Millions	Oct. 01, 2018USD ($)Branchesshares	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)
Subsequent Event [Line Items]
Total loans		$ 31,961.3	$ 32,340.9
Total deposits		33,210.2	$ 33,056.3
First Connecticut Bancorp, Inc. [Member]
Subsequent Event [Line Items]
Merger-related expenses		$ 1.5
First Connecticut Bancorp, Inc. [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Business acquisition, date of acquisition agreement	Oct. 1, 2018
Total loans	$ 2,970
Total deposits	$ 2,400
Number of branches to be acquired | Branches	25
Business acquisition common stock shares | shares	28.4
Business acquisition common stock fair value	$ 486.4