Pension Plans and Other Postretirement Benefits - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Minimum pension liability	$ 465,000,000
Minimum pension liability adjustment net of tax	$ 342,000,000
Increase in minimum liability adjustment		$ 110,000,000
Discount rate	3.25%	4.25%
Defined contribution pension and retirement savings plans total expense	$ 32,000,000	$ 29,000,000	$ 30,000,000
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	8,000,000
Effect of 25 basis point decrease in discount rate on benefit obligation	75,000,000
Effect of 25 basis point increase in discount rate on benefit obligation	$ 79,000,000
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 nine years
Defined benefit pension plans	$ 439,000,000
Percentage of total assets	22.00%
Defined benefit plan fair value of plan assets	$ 2,035,878,000	1,831,773,000
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 contributions	$ 0	0	200,000
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	$ 48,000,000	43,000,000	38,000,000
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	$ 439,185,000	361,178,000
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	$ 148,603,000	$ 125,299,000
Percentage of common stock in total plan assets	7.00%	7.00%
Supplemental Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 154,000,000	$ 143,000,000
Accumulated benefit obligation	154,000,000	143,000,000
Employer contributions	10,000,000	15,000,000
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	2,247,329,000	1,949,613,000	2,188,736,000
Accumulated benefit obligation	2,200,000,000	$ 1,900,000,000
Minimum pension liability	$ 509,000,000
Discount rate	3.25%	4.25%
Defined benefit plan fair value of plan assets	$ 2,037,940,000	$ 1,833,833,000	2,014,891,000
Employer contributions	10,431,000	14,875,000
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	56,492,000	$ 59,991,000	$ 68,637,000
Minimum pension liability	$ 44,000,000
Discount rate	3.25%	4.25%
Employer contributions	$ 6,015,000	$ 7,119,000