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Retirement plans - Narrative (Details) - USD ($) | 6 Months Ended
--- | ---
Jun. 30, 2021 | Jun. 30, 2020
TRP |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total net pension obligations | $ 82,400,000 |
Cash contributions | 0 | $ 0
SERP |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Cash contributions | 1,800,000 | $ 3,200,000
Additional cash payments | 5,100,000 |
Accrued liabilities | TRP |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total net pension obligations | $ 7,800,000 |

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