Regulatory Matters - Capital Ratios and Amounts of Company and its Banking Subsidiaries (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Ratio	4.50%
Ratio	0.060
Ratio	0.080
M&T Bank Corporation [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Amount	$ 10,623,368	$ 10,053,887
Ratio	10.00%	9.73%
Amount	$ 11,873,317	$ 11,303,836
Ratio	0.1117	0.1094
Amount	$ 14,207,937	$ 13,480,612
Ratio	0.1337	0.1305
Amount	$ 11,873,317	$ 11,303,836
Ratio	0.0848	0.0959
M&T Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Amount	$ 11,550,462	$ 10,649,953
Ratio	10.90%	10.34%
Amount	$ 11,550,462	$ 10,649,953
Ratio	0.1090	0.1034
Amount	$ 13,373,416	$ 12,342,834
Ratio	0.1262	0.1199
Amount	$ 11,550,462	$ 10,649,953
Ratio	0.0827	0.0908
Wilmington Trust, N.A. [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Amount	$ 630,574	$ 606,538
Ratio	46.57%	56.35%
Amount	$ 630,574	$ 606,538
Ratio	0.4657	0.5635
Amount	$ 632,506	$ 608,130
Ratio	0.4672	0.5650
Amount	$ 630,574	$ 606,538
Ratio	0.1073	0.1312