Pension Plans and Other Postretirement Benefits - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Minimum pension liability	$ 650,000,000
Minimum pension liability adjustment net of tax	$ 481,000,000
Increase in minimum liability adjustment		$ 186,000,000
Discount rate	2.50%	3.25%
Defined contribution pension and retirement savings plans total expense	$ 35,000,000	$ 32,000,000	$ 29,000,000
Assumptions used calculating net periodic benefit sensitivity rate increase decrease discount rate	0.25%
Effect of 25 basis point increase in rate of return on pension expense	$ 5,000,000
Effect of 25 basis point decrease in rate of return on pension expense	9,000,000
Effect of 25 basis point decrease in discount rate on benefit obligation	86,000,000
Effect of 25 basis point increase in discount rate on benefit obligation	$ 91,000,000
Annual rate of increase in health care cost assumed for next fiscal year	6.25%
Defined Benefit Plan, ultimate health care cost trend rate	5.00%
Period for decreasing the annual rate	over eight years
Defined benefit pension plans	$ 479,000,000
Percentage of total assets	20.00%
Defined benefit plan fair value of plan assets	$ 2,418,702,000	2,035,878,000
Qualified benefit pension plans		0
Percentage of pension plan assets that is not exceeded by investment in security of single Non-US Government of Government agency	10.00%
Employer contribution	$ 300,000,000	$ 0
Percentage of eligible employees contribution for Retirement Savings Plan	50.00%
Percentage of employer matching contribution to employee's contribution	100.00%	75.00%
Maximum employer contribution on Employee's qualified contribution	5.00%	4.50%
Qualified defined contribution plans expense during the period	$ 62,000,000	$ 48,000,000	43,000,000
Equity Securities [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations for plan assets	25.00%
Equity Securities [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations for plan assets	60.00%
Total Debt Securities [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations for plan assets	10.00%
Total Debt Securities [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations for plan assets	65.00%
Other Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	$ 479,393,000	439,185,000
Other Securities [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations for plan assets	5.00%
Other Securities [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations for plan assets	60.00%
M&T Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	$ 111,441,000	$ 148,603,000
Percentage of common stock in total plan assets	5.00%	7.00%
Supplemental Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 165,000,000	$ 154,000,000
Accumulated benefit obligation	165,000,000	154,000,000
Employer contribution	10,000,000	10,000,000
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	2,521,292,000	2,247,329,000	1,949,613,000
Accumulated benefit obligation	2,500,000,000	$ 2,200,000,000
Minimum pension liability	$ 686,000,000
Discount rate	2.50%	3.25%
Defined benefit plan fair value of plan assets	$ 2,420,582,000	$ 2,037,940,000	1,833,833,000
Employer contribution	310,378,000	10,431,000
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	55,281,000	$ 56,492,000	$ 59,991,000
Minimum pension liability	$ 36,000,000
Discount rate	2.50%	3.25%
Employer contribution	$ 6,293,000	$ 6,015,000