Pension Benefits - Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Fair Value of Plan Assets	$ 218,773	$ 205,463	$ 184,758	$ 190,534
Minimum
Pension Benefits
Redemption notice	65 days
Maximum
Pension Benefits
Redemption notice	90 days
Large-Cap Disciplined Equity
Pension Benefits
Fair Value of Plan Assets	$ 8,065	6,844
International Equity
Pension Benefits
Fair Value of Plan Assets	42,178	35,962
Long Duration Fixed Income
Pension Benefits
Fair Value of Plan Assets	138,399	133,592
Other Investments
Pension Benefits
Fair Value of Plan Assets	30,131	29,065
Quoted Prices in Active Markets for Identical Assets (Level 1)
Pension Benefits
Fair Value of Plan Assets	188,642	176,398
Quoted Prices in Active Markets for Identical Assets (Level 1) | Large-Cap Disciplined Equity
Pension Benefits
Fair Value of Plan Assets	8,065	6,844
Quoted Prices in Active Markets for Identical Assets (Level 1) | International Equity
Pension Benefits
Fair Value of Plan Assets	42,178	35,962
Quoted Prices in Active Markets for Identical Assets (Level 1) | Long Duration Fixed Income
Pension Benefits
Fair Value of Plan Assets	138,399	133,592
Quoted Prices in Active Markets for Identical Assets (Level 1) | High Yield
Pension Benefits
Fair Value of Plan Assets		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Emerging Markets Debt
Pension Benefits
Fair Value of Plan Assets		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Investments
Pension Benefits
Fair Value of Plan Assets	0	0
Significant Observable Inputs (Level 2)
Pension Benefits
Fair Value of Plan Assets	0	0
Significant Observable Inputs (Level 2) | Large-Cap Disciplined Equity
Pension Benefits
Fair Value of Plan Assets	0	0
Significant Observable Inputs (Level 2) | International Equity
Pension Benefits
Fair Value of Plan Assets	0	0
Significant Observable Inputs (Level 2) | Long Duration Fixed Income
Pension Benefits
Fair Value of Plan Assets	0	0
Significant Observable Inputs (Level 2) | High Yield
Pension Benefits
Fair Value of Plan Assets		0
Significant Observable Inputs (Level 2) | Emerging Markets Debt
Pension Benefits
Fair Value of Plan Assets		0
Significant Observable Inputs (Level 2) | Other Investments
Pension Benefits
Fair Value of Plan Assets	0	0
Significant Unobservable Inputs (Level 3)
Pension Benefits
Fair Value of Plan Assets	0	0
Significant Unobservable Inputs (Level 3) | Large-Cap Disciplined Equity
Pension Benefits
Fair Value of Plan Assets	0	0
Significant Unobservable Inputs (Level 3) | International Equity
Pension Benefits
Fair Value of Plan Assets	0	0
Significant Unobservable Inputs (Level 3) | Long Duration Fixed Income
Pension Benefits
Fair Value of Plan Assets	0	0
Significant Unobservable Inputs (Level 3) | High Yield
Pension Benefits
Fair Value of Plan Assets		0
Significant Unobservable Inputs (Level 3) | Emerging Markets Debt
Pension Benefits
Fair Value of Plan Assets		0
Significant Unobservable Inputs (Level 3) | Other Investments
Pension Benefits
Fair Value of Plan Assets	$ 0	$ 0