Pension and other postretirement benefits - Fair values of pension plan assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Investment, Type [Extensible Enumeration]	Equity Securities [Member]
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%
Structured Credit | Fair Value, Inputs, Level 1, 2 and 3
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets		$ 29
Structured Credit | 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
Structured Credit | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets
Structured Credit | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets		0
Small and Mid-Sized Companies | 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	$ 285,513	357,270
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	285,513	357,270
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	200,364	287,004
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	72,354	59,152
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Fair value of plan assets	12,795	11,114
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	909	769
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	909	769
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	4,424	13
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	4,424	13
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		46,721
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		46,721
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
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
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		6,054
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		6,054
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
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
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		28,159
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		28,159
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
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
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	193,674	105,865
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	193,674	105,865
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	1,357	87,018
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	1,357	87,018
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		6,092
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		6,092
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
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
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		6,284
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		6,284
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
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
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	72,037	58,572
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	0	0
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	72,037	58,572
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	317	427
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	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	317	427
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	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		153
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
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		153
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
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	12,795	11,114
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	$ 12,795	$ 11,114