Pension Plan and Employee Benefits - Unobservable Input Reconciliation (Details) - PENSION BENEFITS - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation[Roll Forward]
Fair value of plan assets at beginning of period	$ 460,097	$ 391,933
Fair value of plan assets at end of period	516,120	460,097
SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3)
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation[Roll Forward]
Fair value of plan assets at beginning of period	18,017
Fair value of plan assets at end of period	35,962	18,017
Real estate funds
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation[Roll Forward]
Fair value of plan assets at beginning of period	18,017
Fair value of plan assets at end of period	35,962	18,017
Real estate funds \| SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3)
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation[Roll Forward]
Fair value of plan assets at beginning of period	18,017	20,298
Realized gains (losses)	251	370
Unrealized gains (losses)	(1,603)	(1,727)
Purchases	20,893	759
Sales	(1,596)	(1,683)
Fair value of plan assets at end of period	$ 35,962	$ 18,017

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets sold.

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- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets still held.

+ References

+ Details

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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- Definition

Amount, measured using unobservable input, of decrease in plan asset of defined benefit plan from sale.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
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- Definition

Amount, measured using unobservable input, of increase in plan asset of defined benefit plan from purchase.

+ References

+ Details

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