Pension Plan and Employee Benefits - Fair Value of Pension Plan Assets (Details) - PENSION BENEFITS - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Total net assets	$ 516,120	$ 460,097	$ 391,933
Cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total net assets	19,567	4,810
Government securities
Defined Benefit Plan Disclosure [Line Items]
Total net assets	26,863	19,517
Mutual funds, Domestic
Defined Benefit Plan Disclosure [Line Items]
Total net assets	107,055	102,184
Mutual funds, International
Defined Benefit Plan Disclosure [Line Items]
Total net assets	60,104	53,041
Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Total net assets	35,962	18,017
Corporate debt
Defined Benefit Plan Disclosure [Line Items]
Total net assets	192,261	157,109
Interest accrual
Defined Benefit Plan Disclosure [Line Items]
Total net assets	2,264	2,093
Defined Benefit Plan, Common Collective Trust
Defined Benefit Plan Disclosure [Line Items]
Total net assets	$ 72,044	$ 103,326
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List]	us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember	us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember
Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Total net assets	$ 441,812	$ 354,678
QUOTED PRICES IN ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1)
Defined Benefit Plan Disclosure [Line Items]
Total net assets	167,159	155,225
QUOTED PRICES IN ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1) | Cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total net assets	0	0
QUOTED PRICES IN ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1) | Government securities
Defined Benefit Plan Disclosure [Line Items]
Total net assets	0	0
QUOTED PRICES IN ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1) | Mutual funds, Domestic
Defined Benefit Plan Disclosure [Line Items]
Total net assets	107,055	102,184
QUOTED PRICES IN ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1) | Mutual funds, International
Defined Benefit Plan Disclosure [Line Items]
Total net assets	60,104	53,041
QUOTED PRICES IN ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1) | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Total net assets	0	0
QUOTED PRICES IN ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1) | Corporate debt
Defined Benefit Plan Disclosure [Line Items]
Total net assets	0	0
SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2)
Defined Benefit Plan Disclosure [Line Items]
Total net assets	238,691	181,436
SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2) | Cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total net assets	19,567	4,810
SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2) | Government securities
Defined Benefit Plan Disclosure [Line Items]
Total net assets	26,863	19,517
SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2) | Mutual funds, Domestic
Defined Benefit Plan Disclosure [Line Items]
Total net assets	0	0
SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2) | Mutual funds, International
Defined Benefit Plan Disclosure [Line Items]
Total net assets	0	0
SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2) | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Total net assets	0	0
SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2) | Corporate debt
Defined Benefit Plan Disclosure [Line Items]
Total net assets	192,261	157,109
SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3)
Defined Benefit Plan Disclosure [Line Items]
Total net assets	35,962	18,017
SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3) | Cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total net assets	0	0
SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3) | Government securities
Defined Benefit Plan Disclosure [Line Items]
Total net assets	0	0
SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3) | Mutual funds, Domestic
Defined Benefit Plan Disclosure [Line Items]
Total net assets	0	0
SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3) | Mutual funds, International
Defined Benefit Plan Disclosure [Line Items]
Total net assets	0	0
SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3) | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Total net assets	35,962	18,017	$ 20,298
SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3) | Corporate debt
Defined Benefit Plan Disclosure [Line Items]
Total net assets	$ 0	$ 0