Pension Plans and Other Postretirement Benefits - Fair Values of Company's Pension Plan Assets by Asset Category (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	$ 2,013,117,000	$ 1,640,300,000
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	1,532,836,000	1,340,215,000
Significant Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	382,853,000	243,845,000
Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	97,428,000	56,240,000
Money-Market Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	117,648,000	39,556,000
Money-Market Investments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	62,706,000	35,562,000
Money-Market Investments [Member] | Significant Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	54,942,000	3,994,000
M&T Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	154,818,000	164,474,000
M&T Equity Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	154,818,000	164,474,000
Domestic Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	240,763,000	200,595,000
Domestic Equity Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	240,763,000	200,595,000
International Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	13,349,000	14,364,000
International Equity Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	13,349,000	14,364,000
Domestic Mutual Fund Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	205,509,000	250,472,000
Domestic Mutual Fund Equity securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	205,509,000	250,472,000
International Mutual Fund Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	405,200,000	290,172,000
International Mutual Fund Equity Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	405,200,000	290,172,000
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	1,019,639,000	920,077,000
Equity Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	1,019,639,000	920,077,000
Corporate Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	89,751,000	104,909,000
Corporate Debt Securities [Member] | Significant Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	89,751,000	104,909,000
Government Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	235,984,000	121,869,000
Government Debt Securities [Member] | Significant Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	235,984,000	121,869,000
International Debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	2,176,000	13,073,000
International Debt securities [Member] | Significant Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	2,176,000	13,073,000
Domestic Mutual Fund Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	243,456,000	205,847,000
Domestic Mutual Fund Debt Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	243,456,000	205,847,000
Total Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	571,367,000	445,698,000
Total Debt Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	243,456,000	205,847,000
Total Debt Securities [Member] | Significant Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	327,911,000	239,851,000
Other Diversified Mutual Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	80,227,000	92,691,000
Other Diversified Mutual Fund [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	80,227,000	92,691,000
Other Real Estate Partnerships [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	3,747,000	3,112,000
Other Real Estate Partnerships [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	842,000	768,000
Other Real Estate Partnerships [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	2,905,000	2,344,000
Other Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	31,484,000	21,924,000
Other Private Equity [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	31,484,000	21,924,000
Other Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	178,080,000	106,250,000
Other Hedge Funds [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	125,966,000	85,270,000
Other Hedge Funds [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	52,114,000	20,980,000
Guaranteed Deposit Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	10,925,000	10,992,000
Guaranteed Deposit Fund [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	10,925,000	10,992,000
Other Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	304,463,000	234,969,000
Other Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	207,035,000	178,729,000
Other Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of plan assets	$ 97,428,000	$ 56,240,000