Pension Plan and Employee Benefits - Fair Value of Pension Plan Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
PENSION BENEFITS
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	316,730	$ 392,949
Interest accrual	2,103	3,037
Total net assets	391,933	444,089	$ 403,715
PENSION BENEFITS | Cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,471	4,825
PENSION BENEFITS | Common stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13,111	17,655
PENSION BENEFITS | Obligations of Government, Government Agencies, and state and local governments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19,831	50,852
PENSION BENEFITS | Mutual funds, Domestic
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	79,210	58,617
PENSION BENEFITS | Mutual funds, International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	43,418	36,970
PENSION BENEFITS | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20,298	19,195
PENSION BENEFITS | Corporate debt
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	138,391	204,835
PENSION BENEFITS | QUOTED PRICES IN ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	135,739	113,242
PENSION BENEFITS | QUOTED PRICES IN ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1) | Cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
PENSION BENEFITS | QUOTED PRICES IN ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1) | Common stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13,111	17,655
PENSION BENEFITS | QUOTED PRICES IN ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1) | Obligations of Government, Government Agencies, and state and local governments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
PENSION BENEFITS | QUOTED PRICES IN ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1) | Mutual funds, Domestic
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	79,210	58,617
PENSION BENEFITS | QUOTED PRICES IN ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1) | Mutual funds, International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	43,418	36,970
PENSION BENEFITS | QUOTED PRICES IN ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1) | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
PENSION BENEFITS | QUOTED PRICES IN ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1) | Corporate debt
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
PENSION BENEFITS | SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	160,693	260,512
PENSION BENEFITS | SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2) | Cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,471	4,825
PENSION BENEFITS | SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2) | Common stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
PENSION BENEFITS | SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2) | Obligations of Government, Government Agencies, and state and local governments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19,831	50,852
PENSION BENEFITS | SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2) | Mutual funds, Domestic
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
PENSION BENEFITS | SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2) | Mutual funds, International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
PENSION BENEFITS | SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2) | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
PENSION BENEFITS | SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2) | Corporate debt
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	138,391	204,835
PENSION BENEFITS | SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20,298	19,195
PENSION BENEFITS | SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3) | Cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
PENSION BENEFITS | SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3) | Common stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
PENSION BENEFITS | SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3) | Obligations of Government, Government Agencies, and state and local governments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
PENSION BENEFITS | SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3) | Mutual funds, Domestic
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
PENSION BENEFITS | SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3) | Mutual funds, International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
PENSION BENEFITS | SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3) | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20,298	19,195
PENSION BENEFITS | SIGNIFICANT UNOBSERVABLE INPUTS (LEVEL 3) | Corporate debt
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
PENSION BENEFITS | Investments measured at net asset value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 73,100	$ 48,103