Pension Plan and Employee Benefits - Net Periodic Pension and Other Benefits Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
PENSION BENEFITS
Components of periodic benefit costs
Service cost	$ 2,377	$ 2,260	$ 7,130	$ 6,779
Interest cost	5,215	5,412	15,869	16,235
Expected return on plan assets	(5,943)	(6,016)	(17,818)	(18,048)
Amortizations
Prior period service credit	(18)	(18)	(53)	(53)
Net loss (gain)	3,078	2,502	8,998	7,506
Net periodic benefit cost	4,709	4,140	14,126	12,419
OTHER BENEFITS
Components of periodic benefit costs
Service cost	336	314	1,011	1,084
Interest cost	361	372	1,076	1,176
Expected return on plan assets	0	0	0	0
Amortizations
Prior period service credit	0	0	0	0
Net loss (gain)	96	(32)	105	(37)
Net periodic benefit cost	$ 793	$ 654	$ 2,192	$ 2,223