Acquisition - Summary of Allocation of Goodwill (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2022 USD ($)
Business Combination Segment Allocation [Line Items]
Goodwill, Beginning balance	$ 4,593,112
Goodwill, Ending balance	8,501,357
Peoples United Financial Inc [Member]
Business Combination Segment Allocation [Line Items]
Goodwill, Beginning balance	4,593,112
Acquisition of People's United	3,908,245
Goodwill, Ending balance	8,501,357
Business Banking [Member] | Peoples United Financial Inc [Member]
Business Combination Segment Allocation [Line Items]
Goodwill, Beginning balance	864,366
Acquisition of People's United	693,905
Goodwill, Ending balance	1,558,271
Commercial Banking [Member] | Peoples United Financial Inc [Member]
Business Combination Segment Allocation [Line Items]
Goodwill, Beginning balance	1,401,873
Acquisition of People's United	2,686,253
Goodwill, Ending balance	4,088,126
Commercial Real Estate [Member] | Peoples United Financial Inc [Member]
Business Combination Segment Allocation [Line Items]
Goodwill, Beginning balance	654,389
Acquisition of People's United	291,217
Goodwill, Ending balance	945,606
Retail Banking [Member] | Peoples United Financial Inc [Member]
Business Combination Segment Allocation [Line Items]
Goodwill, Beginning balance	1,309,191
Acquisition of People's United	221,196
Goodwill, Ending balance	1,530,387
All Other [Member] | Peoples United Financial Inc [Member]
Business Combination Segment Allocation [Line Items]
Goodwill, Beginning balance	363,293
Acquisition of People's United	15,674
Goodwill, Ending balance	$ 378,967