Goodwill and Other Acquisition-Related Intangible Assets - Schedule of Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Beginning balance	$ 2,685.7	$ 2,411.4
Write-down relating to the sale of branches	(6.9)
Ending balance	3,065.5	2,685.7
First Connecticut Bancorp, Inc. [Member]
Goodwill [Roll Forward]
Acquisition		250.4
Vend Lease [Member]
Goodwill [Roll Forward]
Acquisition		23.9
United Financial [Member]
Goodwill [Roll Forward]
Acquisition	204.3
BSB Bancorp [Member]
Goodwill [Roll Forward]
Acquisition	144.9
VAR Technology Finance [Member]
Goodwill [Roll Forward]
Acquisition	37.5
Commercial Banking Loan [Member]
Goodwill [Roll Forward]
Beginning balance	1,759.4	1,600.3
Write-down relating to the sale of branches	(0.6)
Ending balance	1,966.1	1,759.4
Commercial Banking Loan [Member] | First Connecticut Bancorp, Inc. [Member]
Goodwill [Roll Forward]
Acquisition		135.2
Commercial Banking Loan [Member] | Vend Lease [Member]
Goodwill [Roll Forward]
Acquisition		23.9
Commercial Banking Loan [Member] | United Financial [Member]
Goodwill [Roll Forward]
Acquisition	120.5
Commercial Banking Loan [Member] | BSB Bancorp [Member]
Goodwill [Roll Forward]
Acquisition	49.3
Commercial Banking Loan [Member] | VAR Technology Finance [Member]
Goodwill [Roll Forward]
Acquisition	37.5
Retail Banking [Member]
Goodwill [Roll Forward]
Beginning balance	835.3	720.1
Write-down relating to the sale of branches	(3.8)
Ending balance	1,010.9	835.3
Retail Banking [Member] | First Connecticut Bancorp, Inc. [Member]
Goodwill [Roll Forward]
Acquisition		115.2
Retail Banking [Member] | Vend Lease [Member]
Goodwill [Roll Forward]
Acquisition		0.0
Retail Banking [Member] | United Financial [Member]
Goodwill [Roll Forward]
Acquisition	83.8
Retail Banking [Member] | BSB Bancorp [Member]
Goodwill [Roll Forward]
Acquisition	95.6
Retail Banking [Member] | VAR Technology Finance [Member]
Goodwill [Roll Forward]
Acquisition	0.0
Wealth Management [Member]
Goodwill [Roll Forward]
Beginning balance	91.0	91.0
Write-down relating to the sale of branches	(2.5)
Ending balance	88.5	91.0
Wealth Management [Member] | First Connecticut Bancorp, Inc. [Member]
Goodwill [Roll Forward]
Acquisition		0.0
Wealth Management [Member] | Vend Lease [Member]
Goodwill [Roll Forward]
Acquisition		$ 0.0
Wealth Management [Member] | United Financial [Member]
Goodwill [Roll Forward]
Acquisition	0.0
Wealth Management [Member] | BSB Bancorp [Member]
Goodwill [Roll Forward]
Acquisition	0.0
Wealth Management [Member] | VAR Technology Finance [Member]
Goodwill [Roll Forward]
Acquisition	$ 0.0