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Pension and other post-retirement benefits - Fair Value of Investments by Asset Category (Details) - Level 1 $ in Thousands | Dec. 31, 2020 USD ($)
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Defined Benefit Plan Disclosure [Line Items] |
Allocation percentage | 100.00%
Fair value of plan assets | $ 805,773
Equity securities |
Defined Benefit Plan Disclosure [Line Items] |
Allocation percentage | 59.00%
Fair value of plan assets | $ 479,506
Debt securities |
Defined Benefit Plan Disclosure [Line Items] |
Allocation percentage | 32.00%
Fair value of plan assets | $ 255,975
Other |
Defined Benefit Plan Disclosure [Line Items] |
Allocation percentage | 9.00%
Fair value of plan assets | $ 70,292

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