Regulatory Matters - Capital Ratios and Amounts of Company and its Banking Subsidiaries (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Ratio	4.50%
Ratio	6.00%
Ratio	8.00%
M&T Bank Corporation [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Amount	$ 10,053,887	$ 9,960,811
Ratio	9.73%	10.13%
Amount	$ 11,303,836	$ 11,193,770
Ratio	10.94%	11.38%
Amount	$ 13,480,612	$ 13,454,137
Ratio	13.05%	13.68%
Amount	$ 11,303,836	$ 11,193,770
Ratio	9.59%	9.88%
M&T Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Amount	$ 10,649,953	$ 10,636,136
Ratio	10.34%	10.84%
Amount	$ 10,649,953	$ 10,636,136
Ratio	10.34%	10.84%
Amount	$ 12,342,834	$ 12,475,296
Ratio	11.99%	12.72%
Amount	$ 10,649,953	$ 10,636,136
Ratio	9.08%	9.42%
Wilmington Trust, N.A. [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Amount	$ 606,538	$ 585,767
Ratio	56.35%	60.69%
Amount	$ 606,538	$ 585,767
Ratio	56.35%	60.69%
Amount	$ 608,130	$ 589,671
Ratio	56.50%	61.10%
Amount	$ 606,538	$ 585,767
Ratio	13.12%	12.51%