Acquisitions - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended							1 Months Ended			3 Months Ended	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Aug. 27, 2012	Dec. 31, 2011	Sep. 30, 2012 Hudson City [Member]	Aug. 27, 2012 Cash [Member]	Aug. 27, 2012 Common stock [Member]	May 31, 2012 Minimum [Member]	May 31, 2011 Maximum [Member]	May 31, 2011 Wilmington Trust Corporation [Member]	Mar. 31, 2011 Wilmington Trust Corporation [Member]	Dec. 31, 2011 Wilmington Trust Corporation [Member]	May 16, 2011 Wilmington Trust Corporation [Member] Office
Business Acquisition [Line Items]
Total consideration in shares					$ 0.08403
Percentage of total consideration								40.00%	60.00%
Assets							$ 41,900,000,000
Loans							27,800,000,000
Investment securities	6,624,004,000		6,624,004,000			7,673,154,000	12,500,000,000
Liabilities	71,139,706,000		71,139,706,000			68,653,078,000	37,200,000,000
Deposits	64,007,258,000		64,007,258,000			59,394,649,000	24,000,000,000
Long-term borrowing							13,000,000,000
Number of banking offices operated															55
Shares of M&T common stock exchanged for each share of Wilmington Trust common stock															0.051372
Common stock issued for acquisition												4,694,486
Common shares issued for acquisition fair value															406,000,000
Assets acquired total															10,845,042,000
Loans and leases															6,410,430,000
Commercial real estate loans															3,200,000,000
Commercial loans and leases															1,400,000,000
Consumer loans															1,100,000,000
Residential real estate loans															680,000,000
Liabilities															10,044,555,000
Deposits															8,864,161,000
Value added in shareholders equity due to acquisition															406,000,000
Preferred stock redeemed in connection with Willington Trust acquisition															330,000,000
Core deposit and other intangibles															112,000,000
Intangible Assets Amortization										5 years	7 years
Non taxable gain recorded in connection with acquisition												65,000,000
Provision for credit losses	46,000,000	58,000,000	155,000,000	196,000,000									41,000,000
Other- than-temporary impairment														$ 5,000,000