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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation Of Revenue [Line Items]
Brokerage services income	$ 12,629	$ 16,617	$ 26,021	$ 34,001
Other revenues from operations:
Total other income	457,414	460,816	916,110	907,661
Service Charges on Deposit Accounts [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contract with customer	106,784	106,057	211,899	210,233
Trust Income [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contract with customer	137,641	$ 126,797	269,016	$ 246,812
ASU 2014-09 [Member]
Disaggregation Of Revenue [Line Items]
Brokerage services income	12,629		26,021
Other revenues from operations:
Merchant discount and credit card fees	26,062		50,834
Other	24,412		47,302
Total other income	307,528		605,072
ASU 2014-09 [Member] | Service Charges on Deposit Accounts [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contract with customer	106,784		211,899
ASU 2014-09 [Member] | Trust Income [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contract with customer	137,641		269,016
ASU 2014-09 [Member] | Business Banking [Member]
Other revenues from operations:
Merchant discount and credit card fees	8,334		15,837
Total other income	23,945		46,852
ASU 2014-09 [Member] | Business Banking [Member] | Service Charges on Deposit Accounts [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contract with customer	15,611		31,015
ASU 2014-09 [Member] | Commercial Banking [Member]
Other revenues from operations:
Merchant discount and credit card fees	12,542		25,254
Other	3,646		5,209
Total other income	40,458		79,387
ASU 2014-09 [Member] | Commercial Banking [Member] | Service Charges on Deposit Accounts [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contract with customer	24,270		48,924
ASU 2014-09 [Member] | Commercial Real Estate [Member]
Other revenues from operations:
Merchant discount and credit card fees	428		982
Other	2,215		3,074
Total other income	5,094		9,271
ASU 2014-09 [Member] | Commercial Real Estate [Member] | Service Charges on Deposit Accounts [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contract with customer	2,451		5,215
ASU 2014-09 [Member] | Discretionary Portfolio [Member]
Other revenues from operations:
Other	413		865
Total other income	413		865
ASU 2014-09 [Member] | Residential Mortgage Banking [Member]
Other revenues from operations:
Other	927		1,970
Total other income	930		1,976
ASU 2014-09 [Member] | Residential Mortgage Banking [Member] | Service Charges on Deposit Accounts [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contract with customer	3		6
ASU 2014-09 [Member] | Retail Banking [Member]
Other revenues from operations:
Merchant discount and credit card fees	4,140		7,528
Other	9,613		19,478
Total other income	76,768		150,622
ASU 2014-09 [Member] | Retail Banking [Member] | Service Charges on Deposit Accounts [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contract with customer	63,015		123,616
ASU 2014-09 [Member] | All Other [Member]
Disaggregation Of Revenue [Line Items]
Brokerage services income	12,629		26,021
Other revenues from operations:
Merchant discount and credit card fees	618		1,233
Other	7,598		16,706
Total other income	159,920		316,099
ASU 2014-09 [Member] | All Other [Member] | Service Charges on Deposit Accounts [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contract with customer	1,434		3,123
ASU 2014-09 [Member] | All Other [Member] | Trust Income [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contract with customer	$ 137,641		$ 269,016