Segment Information - Information about Company's Segments (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Taxable-equivalent adjustment	$ 11,827,000	$ 3,703,000		$ 25,787,000	$ 11,168,000
Net income	646,596	495,460		1,226,292	1,400,778
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net income	$ 214,063	$ 144,367		521,749	$ 378,040
Peoples United Financial Inc [Member]
Segment Reporting Information [Line Items]
Revenues			$ 1,000,000	96,000
Net income			$ 13,000	$ 32,600