Employee benefits - Plan assets by fair value hierarchy (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 617,175	$ 652,426
Government, its agencies, states and political subdivisions
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	132,432	158,170
Corporate bonds and debentures
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	375,634	302,183
Equity securities - Common stocks
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,821	34,334
Equity securities - ETF's
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	38,875	59,971
Foreign Commingled Trust Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20,097	51,392
Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9,833	24,252
Mortgage Backed Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	14,160	9,289
Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17,034	8,908
Accrued investment income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,289	3,927
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	53,227	86,040
Level 1 | Government, its agencies, states and political subdivisions
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 1 | Corporate bonds and debentures
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 1 | Equity securities - Common stocks
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,821	34,334
Level 1 | Equity securities - ETF's
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	32,372	42,798
Level 1 | Foreign Commingled Trust Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 1 | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 1 | Mortgage Backed Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 1 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17,034	8,908
Level 1 | Accrued investment income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	402,352	326,924
Level 2 | Government, its agencies, states and political subdivisions
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,956	3,711
Level 2 | Corporate bonds and debentures
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	364,900	295,141
Level 2 | Equity securities - Common stocks
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Equity securities - ETF's
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,503	17,173
Level 2 | Foreign Commingled Trust Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9,833	1,610
Level 2 | Mortgage Backed Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	14,160	9,289
Level 2 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Accrued investment income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,289	3,927	$ 3,581
Level 3 | Government, its agencies, states and political subdivisions
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Corporate bonds and debentures
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Equity securities - Common stocks
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Equity securities - ETF's
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Foreign Commingled Trust Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Mortgage Backed Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Accrued investment income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,289	3,927
NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	156,307	235,535
NAV | Government, its agencies, states and political subdivisions
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	125,476	154,459
NAV | Corporate bonds and debentures
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10,734	7,042
NAV | Equity securities - Common stocks
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
NAV | Equity securities - ETF's
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
NAV | Foreign Commingled Trust Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20,097	51,392
NAV | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	22,642
NAV | Mortgage Backed Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
NAV | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
NAV | Accrued investment income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0