Income Taxes - Additional Information (Detail) $ in Millions	1 Months Ended	3 Months Ended									12 Months Ended
	Feb. 28, 2018 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Nov. 01, 2019 funds mobileSolarUnit
Schedule Of Income Tax Expense [Line Items]
Income tax benefit										$ (6.5)
Deferred tax assets										58.0
Deferred tax liabilities										$ 64.5
Income tax expense (benefit)		$ 36.4	$ 30.4	$ 35.2	$ 30.0	$ 15.8	$ 32.2	$ 30.8	$ 29.4		$ 132.0	$ 108.2	$ 129.9
Employer contributions	$ 50.0
Affordable housing investments		383.9				304.1					383.9	304.1
Future contingent commitments		141.1				119.7					$ 141.1	119.7
Amortization period											10 years
Income tax expense											$ 25.0	19.1	16.7
Number of tax advantage funds | funds														3
Number of mobile solar generators to be acquired under limited liability company | mobileSolarUnit														500
Connecticut tax net operating loss carryforwards		1,200.0									1,200.0
Current income tax receivable		51.7				31.4					51.7	31.4
Accrued interest expense related to the unrecognized income tax benefits		$ 0.1				0.9					0.1	0.9
Pension Plans [Member]
Schedule Of Income Tax Expense [Line Items]
Income tax expense (benefit)						$ (9.2)
Employer contributions											18.7	52.7
People's United Financial, Inc. [Member]
Schedule Of Income Tax Expense [Line Items]
Income tax expense (benefit)											(6.2)	(5.5)	(5.7)
People's United Financial, Inc. [Member] | D.C. Solar investment [Member]
Schedule Of Income Tax Expense [Line Items]
Write down of investment											8.4
Additions related to D.C. Solar											$ 18.9	$ 0.0	$ 0.0