Pension Plan and Employee Benefits - Amounts Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
PENSION BENEFITS
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain) occurring during period	$ 9,722	$ 7,434
Net actuarial loss (gain) amortized during period	12,313	10,008
Prior service (credit) cost amortized during period	(71)	(71)
Expected net actuarial loss	7,496
Expected prior service credit	(71)
Net actuarial loss (gain)	140,377	142,967
Prior service credit	(131)	(203)
OTHER BENEFITS
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain) occurring during period	(1,106)	437
Net actuarial loss (gain) amortized during period	135	(50)
Prior service (credit) cost amortized during period	0	0
Expected net actuarial loss	(151)
Expected prior service credit	0
Net actuarial loss (gain)	1,814	2,779
Prior service credit	0	0
SERP BENEFITS
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain) occurring during period	(5,163)	6,622
Net actuarial loss (gain) amortized during period	2,913	2,105
Prior service (credit) cost amortized during period	(160)	(190)
Expected net actuarial loss	1,729
Expected prior service credit	(160)
Net actuarial loss (gain)	17,261	25,336
Prior service credit	$ (1,837)	$ (1,997)