Pension and other postretirement benefits - Fair values of pension plan assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Maximum percentage of net assets invested in emerging market	35.00%
Minimum
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Effective average duration to maintain	3 years
Maximum
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Effective average duration to maintain	10 years
Small and Mid-Sized Companies | Equity Securities | U.S. Russell 2500 Index
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Percentage of net assets invested	80.00%
Foreign Companies
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Percentage of net assets invested in foreign equity securities	80.00%
Fixed Income Securities
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Percentage of net assets invested	80.00%
Fair Value, Measurements, Recurring
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	$ 457,626	$ 423,300
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	429,882	387,430
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	414,299	373,773
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	5,094	3,803
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	10,489	9,854
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	27,744	35,870
Fair Value, Measurements, Recurring | Cash | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	582	650
Fair Value, Measurements, Recurring | Cash | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	582	650
Fair Value, Measurements, Recurring | Cash | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Cash | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Money market funds | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	12	5
Fair Value, Measurements, Recurring | Money market funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	12	5
Fair Value, Measurements, Recurring | Money market funds | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Money market funds | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Managed Volatility | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	85,974	72,334
Fair Value, Measurements, Recurring | Managed Volatility | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	85,974	72,334
Fair Value, Measurements, Recurring | Managed Volatility | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Managed Volatility | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | U.S. Small/Mid-Cap Equity | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	11,780	10,014
Fair Value, Measurements, Recurring | U.S. Small/Mid-Cap Equity | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	11,780	10,014
Fair Value, Measurements, Recurring | U.S. Small/Mid-Cap Equity | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | U.S. Small/Mid-Cap Equity | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | World equity (excluding U.S.) | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	59,467	48,285
Fair Value, Measurements, Recurring | World equity (excluding U.S.) | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	59,467	48,285
Fair Value, Measurements, Recurring | World equity (excluding U.S.) | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | World equity (excluding U.S.) | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Common equity securities | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	29,592	38,359
Fair Value, Measurements, Recurring | Common equity securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	29,592	38,359
Fair Value, Measurements, Recurring | Common equity securities | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Common equity securities | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Intermediate duration bond fund | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	63,376	38,500
Fair Value, Measurements, Recurring | Intermediate duration bond fund | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	63,376	38,500
Fair Value, Measurements, Recurring | Intermediate duration bond fund | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Intermediate duration bond fund | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Long duration bond fund | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	98,996	107,143
Fair Value, Measurements, Recurring | Long duration bond fund | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	98,996	107,143
Fair Value, Measurements, Recurring | Long duration bond fund | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Long duration bond fund | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Corporate bond fund | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	13,469	13,107
Fair Value, Measurements, Recurring | Corporate bond fund | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	13,469	13,107
Fair Value, Measurements, Recurring | Corporate bond fund | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Corporate bond fund | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Global credit fund | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets		929
Fair Value, Measurements, Recurring | Global credit fund | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets		929
Fair Value, Measurements, Recurring | Global credit fund | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets		0
Fair Value, Measurements, Recurring | Global credit fund | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets		0
Fair Value, Measurements, Recurring | Emerging markets debt fund | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	11,412	9,974
Fair Value, Measurements, Recurring | Emerging markets debt fund | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	11,412	9,974
Fair Value, Measurements, Recurring | Emerging markets debt fund | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Emerging markets debt fund | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Corporate, government and foreign bonds | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	35,582	29,714
Fair Value, Measurements, Recurring | Corporate, government and foreign bonds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	35,582	29,714
Fair Value, Measurements, Recurring | Corporate, government and foreign bonds | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Corporate, government and foreign bonds | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Asset backed – home loans | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	261	316
Fair Value, Measurements, Recurring | Asset backed – home loans | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Asset backed – home loans | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	261	316
Fair Value, Measurements, Recurring | Asset backed – home loans | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Multi asset funds | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	8,890	8,246
Fair Value, Measurements, Recurring | Multi asset funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	4,057	4,759
Fair Value, Measurements, Recurring | Multi asset funds | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	4,833	3,487
Fair Value, Measurements, Recurring | Multi asset funds | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Contract with insurance company | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	10,485	9,849
Fair Value, Measurements, Recurring | Contract with insurance company | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Contract with insurance company | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Contract with insurance company | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	10,485	9,849
Fair Value, Measurements, Recurring | Other | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	4	5
Fair Value, Measurements, Recurring | Other | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Other | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	0	0
Fair Value, Measurements, Recurring | Other | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	$ 4	$ 5