Regulatory Matters - Capital Ratios and Amounts of Company and its Banking Subsidiaries (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Ratio	4.50%
Ratio	6.00%
Ratio	8.00%
M&T (Consolidated) [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Amount	$ 10,675,735	$ 10,849,642
Ratio	10.99%	10.70%
Amount	$ 11,908,166	$ 12,083,948
Ratio	12.26%	11.92%
Amount	$ 14,328,467	$ 14,282,492
Ratio	14.75%	14.09%
Amount	$ 11,908,166	$ 12,083,948
Ratio	10.31%	9.99%
M&T Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Amount	$ 9,978,163	$ 10,115,688
Ratio	10.30%	10.02%
Amount	$ 9,978,163	$ 10,115,688
Ratio	10.30%	10.02%
Amount	$ 12,012,171	$ 11,812,114
Ratio	12.40%	11.70%
Amount	$ 9,978,163	$ 10,115,688
Ratio	8.68%	8.41%
Wilmington Trust, N.A. [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Amount	$ 529,988	$ 496,801
Ratio	48.16%	57.08%
Amount	$ 529,988	$ 496,801
Ratio	48.16%	57.08%
Amount	$ 534,235	$ 501,111
Ratio	48.54%	57.57%
Amount	$ 529,988	$ 496,801
Ratio	13.03%	15.31%