Revenue from Contracts with Customers - Summary of Sources of M&T's Noninterest Income that are Subject to the Amended Guidance (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation Of Revenue [Line Items]
Service charges on deposit accounts	$ 105,115	$ 104,176
Trust income	131,375	120,015
Brokerage services income	13,392	17,384
Other revenues from operations:
Total other income	458,696	$ 446,845
ASU 2014-09 [Member]
Disaggregation Of Revenue [Line Items]
Service charges on deposit accounts	105,115
Trust income	131,375
Brokerage services income	13,392
Other revenues from operations:
Merchant discount and credit card fees	24,772
Other	22,890
Total other income	297,544
ASU 2014-09 [Member] | Business Banking [Member]
Disaggregation Of Revenue [Line Items]
Service charges on deposit accounts	15,404
Other revenues from operations:
Merchant discount and credit card fees	7,503
Total other income	22,907
ASU 2014-09 [Member] | Commercial Banking [Member]
Disaggregation Of Revenue [Line Items]
Service charges on deposit accounts	24,654
Other revenues from operations:
Merchant discount and credit card fees	12,712
Other	1,563
Total other income	38,929
ASU 2014-09 [Member] | Commercial Real Estate [Member]
Disaggregation Of Revenue [Line Items]
Service charges on deposit accounts	2,764
Other revenues from operations:
Merchant discount and credit card fees	554
Other	859
Total other income	4,177
ASU 2014-09 [Member] | Discretionary Portfolio [Member]
Other revenues from operations:
Other	452
Total other income	452
ASU 2014-09 [Member] | Residential Mortgage Banking [Member]
Disaggregation Of Revenue [Line Items]
Service charges on deposit accounts	3
Other revenues from operations:
Other	1,043
Total other income	1,046
ASU 2014-09 [Member] | Retail Banking [Member]
Disaggregation Of Revenue [Line Items]
Service charges on deposit accounts	60,601
Other revenues from operations:
Merchant discount and credit card fees	3,388
Other	9,865
Total other income	73,854
ASU 2014-09 [Member] | All Other [Member]
Disaggregation Of Revenue [Line Items]
Service charges on deposit accounts	1,689
Trust income	131,375
Brokerage services income	13,392
Other revenues from operations:
Merchant discount and credit card fees	615
Other	9,108
Total other income	$ 156,179