Employee benefits - Plan assets by fair value hierarchy (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 619,885	$ 860,484
Obligations of U.S. Government sponsored entities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	138,510	197,636
Corporate bonds and debentures
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	274,932	384,360
Equity securities - Common stocks
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	32,906	41,414
Equity securities - ETF's
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	72,112	136,811
Foreign Commingled Trust Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	64,630	82,912
Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	25,577	5,262
Private Equity investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	56
Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,637	7,523
Accrued Income Receivable
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,581	4,510
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	92,379	160,302
Level 1 | Obligations of U.S. Government sponsored entities
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	32,906	41,414
Level 1 | Equity securities - ETF's
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	51,836	111,365
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 | Private Equity investments
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	7,637	7,523
Level 1 | Accrued Income Receivable
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	300,501	415,842
Level 2 | Obligations of U.S. Government sponsored entities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,113	9,259
Level 2 | Corporate bonds and debentures
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	268,641	375,875
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	20,276	25,446
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	3,471	5,262
Level 2 | Private Equity investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Accrued Income Receivable
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	3,581	4,566	$ 3,917
Level 3 | Obligations of U.S. Government sponsored entities
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 | Private Equity investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	56
Level 3 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Accrued Income Receivable
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,581	4,510
NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	223,424	279,774
NAV | Obligations of U.S. Government sponsored entities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	130,397	188,377
NAV | Corporate bonds and debentures
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,291	8,485
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	64,630	82,912
NAV | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22,106	0
NAV | Private Equity investments
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 Income Receivable
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0