Pension plans and other postretirement benefits - Fair Values of Company's Pension Plan Assets by Asset Category (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	$ 1,404,138	$ 1,113,566
Money-market funds [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	46,657	63,842
M&T Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	120,914	93,740
Domestic Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	147,382	151,235
International Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	8,561	8,264
Domestic Mutual Fund Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	188,950	125,223
International Mutual Fund Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	208,696	127,071
Total Equity Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	674,503	505,533
Corporate Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	307,712	275,079
Government Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	141,224	88,261
International Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	10,230	7,586
Domestic Mutual Fund Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	107,576	72,680
International Mutual Fund Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	54,362	23,234
Total Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	621,104	466,840
Other Diversified mutual fund [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	10,415	17,557
Other Private real estate [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	4,603	4,677
Other Private equity [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	5,347	10,190
Other Hedge funds [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	41,509	44,927
Other Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	61,874	77,351
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	903,513	682,846
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Money-market funds [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	46,657	63,842
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | M&T Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	120,914	93,740
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Domestic Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	147,382	151,235
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | International Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	8,561	8,264
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Domestic Mutual Fund Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	188,950	125,223
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | International Mutual Fund Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	208,696	127,071
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Total Equity Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	674,503	505,533
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Corporate Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Government Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | International Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Domestic Mutual Fund Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	107,576	72,680
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | International Mutual Fund Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	54,362	23,234
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Total Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	161,938	95,914
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other Diversified mutual fund [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	10,415	17,557
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other Private real estate [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other Private equity [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other Hedge funds [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	10,000
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	20,415	17,557
Significant Observable Inputs (Level 2) [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	459,166	370,926
Significant Observable Inputs (Level 2) [Member] | Money-market funds [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Observable Inputs (Level 2) [Member] | M&T Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Observable Inputs (Level 2) [Member] | Domestic Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Observable Inputs (Level 2) [Member] | International Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Observable Inputs (Level 2) [Member] | Domestic Mutual Fund Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Observable Inputs (Level 2) [Member] | International Mutual Fund Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Observable Inputs (Level 2) [Member] | Total Equity Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Observable Inputs (Level 2) [Member] | Corporate Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	307,712	275,079
Significant Observable Inputs (Level 2) [Member] | Government Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	141,224	88,261
Significant Observable Inputs (Level 2) [Member] | International Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	10,230	7,586
Significant Observable Inputs (Level 2) [Member] | Domestic Mutual Fund Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Observable Inputs (Level 2) [Member] | International Mutual Fund Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Observable Inputs (Level 2) [Member] | Total Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	459,166	370,926
Significant Observable Inputs (Level 2) [Member] | Other Diversified mutual fund [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Observable Inputs (Level 2) [Member] | Other Private real estate [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Observable Inputs (Level 2) [Member] | Other Private equity [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Observable Inputs (Level 2) [Member] | Other Hedge funds [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Observable Inputs (Level 2) [Member] | Other Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Unobservable Inputs (Level 3) [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	41,459	59,794
Significant Unobservable Inputs (Level 3) [Member] | Money-market funds [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Unobservable Inputs (Level 3) [Member] | M&T Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Unobservable Inputs (Level 3) [Member] | Domestic Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Unobservable Inputs (Level 3) [Member] | International Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Unobservable Inputs (Level 3) [Member] | Domestic Mutual Fund Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Unobservable Inputs (Level 3) [Member] | International Mutual Fund Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Unobservable Inputs (Level 3) [Member] | Total Equity Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Unobservable Inputs (Level 3) [Member] | Corporate Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Unobservable Inputs (Level 3) [Member] | Government Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Unobservable Inputs (Level 3) [Member] | International Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Unobservable Inputs (Level 3) [Member] | Domestic Mutual Fund Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Unobservable Inputs (Level 3) [Member] | International Mutual Fund Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Unobservable Inputs (Level 3) [Member] | Total Debt securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Unobservable Inputs (Level 3) [Member] | Other Diversified mutual fund [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets
Significant Unobservable Inputs (Level 3) [Member] | Other Private real estate [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	4,603	4,677
Significant Unobservable Inputs (Level 3) [Member] | Other Private equity [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	5,347	10,190
Significant Unobservable Inputs (Level 3) [Member] | Other Hedge funds [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	31,509	44,927
Significant Unobservable Inputs (Level 3) [Member] | Other Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined benefit plan fair value of plan assets	$ 41,459	$ 59,794