Pension plans and other postretirement benefits - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended
	May 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Minimum pension liability		$ 455,590,000
Minimum pension liability adjustment net of tax		276,771,000
Discount rate of benefit plan	5.25%	3.75%	4.25%
Decrease in minimum liability adjustment			1,555,000
Increase in minimum pension liability adjustment net of tax			945,000
Defined contribution pension and retirement savings plans total expense		17,000,000	13,000,000	14,000,000
Percentage of compensation increased	4.00%
Expected return on assets	6.50%
Annual rate of increase in health care cost assumed for next fiscal year		8.00%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate		5.00%
Period for decreasing the annual rate		Over 30 years
Defined benefit pension plans		61,874,000
Percentage of total assets		4.00%
Defined benefit plan fair value of plan assets		1,404,138,000	1,113,566,000
Qualified benefit pension plans		200,000,000	70,000,000
Percentage of eligible employees contribution for Retirement Savings Plan		50.00%
Percentage of employer matching contribution to employee's contribution		75.00%
Maximum employer contribution on Employee's qualified contribution		4.50%
Qualified defined contribution plans expense during the period		31,305,000	27,738,000	24,683,000
Total Equity Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Target allocations for plan assets, minimum range		55.00%
Target allocations for plan assets, maximum range		70.00%
Defined benefit plan fair value of plan assets		674,503,000	505,533,000
Total Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Target allocations for plan assets, minimum range		25.00%
Target allocations for plan assets, maximum range		40.00%
Defined benefit plan fair value of plan assets		621,104,000	466,840,000
Other Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Target allocations for plan assets, minimum range		3.00%
Target allocations for plan assets, maximum range		10.00%
Defined benefit plan fair value of plan assets		61,874,000	77,351,000
M&T Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets		120,914,000	93,740,000
Percentage of common stock in total plan assets		8.60%	8.40%
Pension benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Projected benefit obligation		1,644,567,000	1,496,490,000	947,993,000
Accumulated benefit obligation		1,618,119,000	1,472,140,000
Accumulated benefit obligation for defined benefit pension plans in which such obligation exceeded plan assets totaled			1,422,958,000
Minimum pension liability		456,629,000
Discount rate of benefit plan		4.25%	5.25%	5.75%
Percentage of compensation increased		4.50%	4.50%	4.50%
Defined benefit plan fair value of plan assets		1,408,771,000	1,117,972,000	838,465,000
Employer contributions		207,115,000	75,600,000
Supplemental pension plan [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Projected benefit obligation		130,745,000	124,877,000
Accumulated benefit obligation		130,745,000	123,085,000
Accumulated benefit obligation for defined benefit pension plans in which such obligation exceeded plan assets totaled			123,085,000
Employer contributions		7,115,000	5,600,000
Other postretirement benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Projected benefit obligation		74,966,000	90,104,000	61,675,000
Minimum pension liability		(1,039,000)
Discount rate of benefit plan		4.25%	5.25%	5.75%
Percentage of compensation increased
Defined benefit plan fair value of plan assets
Employer contributions		$ 6,048,000	$ 6,859,000