Pension Plan and Employee Benefits, Additional Disclosures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
SERP Benefits [Member]
Projected benefit payments [Abstract]
2018	$ 4,553
2019	4,585
2020	4,665
2021	4,672
2022	4,645
Next five years	$ 23,830
Other Postretirement Benefits Plan [Member]
Impact of future Medicare subsidies on the components of other benefit costs [Abstract]
Assumed health care cost trend rates for next fiscal year(in hundredths)	5.00%	5.00%
Ultimate health care trend rate (in hundredths)	5.00%
Effect of one-percentage point change in assumed health care cost trend rates [Abstract]
Effect of one-percentage point increase on total of service and interest cost components	$ 16
Effect of one-percentage point decrease on total of service and interest cost components	(18)
Effect of one-percentage point increase on postretirement benefit obligation	199
Effect of one-percentage point decrease on postretirement benefit obligation	(222)
Projected benefit payments [Abstract]
2018	4,131
2019	4,077
2020	3,979
2021	3,894
2022	3,801
Next five years	16,842
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated pension contributions required through 2022	$ 8,300
Actual return on plan assets (in hundredths)	16.32%	10.90%
Expected return on plan assets (in hundredths)	6.08%	6.21%
Projected benefit payments [Abstract]
2018	$ 21,655
2019	22,795
2020	23,948
2021	25,041
2022	26,189
Next five years	$ 147,475