Pension Plan and Employee Benefits - Pension Plan and Other Benefits Plan (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
PENSION BENEFITS
Components of periodic benefit costs
Service cost	$ 2,361	$ 2,295	$ 4,753	$ 4,520
Interest cost	5,472	5,467	10,654	10,824
Expected return on plan assets	(5,938)	(5,895)	(11,875)	(12,032)
Amortizations
Prior period service credit	(18)	(18)	(36)	(36)
Net loss (gain)	2,960	2,591	5,921	5,004
Net periodic benefit cost	4,837	4,440	9,417	8,280
OTHER BENEFITS
Components of periodic benefit costs
Service cost	338	385	675	770
Interest cost	357	402	715	805
Expected return on plan assets	0	0	0	0
Amortizations
Prior period service credit	0	0	0	0
Net loss (gain)	5	(2)	10	(5)
Net periodic benefit cost	$ 700	$ 785	$ 1,400	$ 1,570