Regulatory Matters - Capital Ratios and Amounts of Company and its Banking Subsidiaries (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
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	$ 9,960,811	$ 10,675,735
Ratio	10.13%	10.99%
Amount	$ 11,193,770	$ 11,908,166
Ratio	11.38%	12.26%
Amount	$ 13,454,137	$ 14,328,467
Ratio	13.68%	14.75%
Amount	$ 11,193,770	$ 11,908,166
Ratio	9.88%	10.31%
M&T Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Amount	$ 10,636,136	$ 9,978,163
Ratio	10.84%	10.30%
Amount	$ 10,636,136	$ 9,978,163
Ratio	10.84%	10.30%
Amount	$ 12,475,296	$ 12,012,171
Ratio	12.72%	12.40%
Amount	$ 10,636,136	$ 9,978,163
Ratio	9.42%	8.68%
Wilmington Trust, N.A. [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Amount	$ 585,767	$ 529,988
Ratio	60.69%	48.16%
Amount	$ 585,767	$ 529,988
Ratio	60.69%	48.16%
Amount	$ 589,671	$ 534,235
Ratio	61.10%	48.54%
Amount	$ 585,767	$ 529,988
Ratio	12.51%	13.03%