Pension Plan and Employee Benefits - Amounts Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
PENSION BENEFITS
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain) occurring during period	$ 30,126	$ 19,075
Net actuarial loss amortized during period	16,292	7,849
Prior service credit amortized during period	(60)	(71)
Net actuarial loss	165,437	151,603
Prior service credit	0	(60)
OTHER BENEFITS
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain) occurring during period	4,221	13,897
Net actuarial loss amortized during period	1,389	21
Prior service credit amortized during period	0	0
Net actuarial loss	21,342	15,732
Prior service credit	0	0
SERP BENEFITS
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain) occurring during period	9,621	11,608
Net actuarial loss amortized during period	3,185	1,544
Prior service credit amortized during period	(215)	(160)
Net actuarial loss	34,825	28,731
Prior service credit	$ (1,728)	$ (1,678)