Pension and other postretirement benefits - Fair values of pension plan assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
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
Emerging markets debt fund
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Percentage of net assets invested in foreign equity securities	80.00%
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%
Fair Value, Measurements, Recurring
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	$ 469,793	$ 457,626
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	459,105	429,882
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	439,093	414,299
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	879	5,094
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	19,133	10,489
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	10,688	27,744
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	923	582
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	923	582
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	6	12
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	6	12
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	57,252	85,974
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	57,252	85,974
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	7,532	11,780
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	7,532	11,780
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	34,287	59,467
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	34,287	59,467
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 | 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	101,363	63,376
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	101,363	63,376
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	171,919	98,996
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	171,919	98,996
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	7,607	13,469
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	7,607	13,469
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 | 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	7,605	11,412
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	7,605	11,412
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	50,599	35,582
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	50,599	35,582
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 | Absolute return 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	671
Fair Value, Measurements, Recurring | Absolute return 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	0
Fair Value, Measurements, Recurring | Absolute return credit fund | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	671
Fair Value, Measurements, Recurring | Absolute return 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 | 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	208	261
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	208	261
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 | 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	19,130	10,485
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	19,130	10,485
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	3	4
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	$ 3	4
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
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
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
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
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
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
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
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