Pension Plan and Employee Benefits - Benefit Obligation, Plan Assets, and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
PENSION BENEFITS
Change in benefit obligation
Benefit obligation at beginning of period	$ 567,215	$ 512,785
Service cost	9,507	9,039
Interest cost	20,860	21,648
Plan participants’ contributions	0	0
Actuarial (gain) loss	(42,935)	46,686
Expenses paid	(2,786)	(3,020)
Benefits paid	(20,925)	(19,923)
Benefit obligation at end of period	530,936	567,215
Change in plan assets
Fair value of plan assets at beginning of period	444,089	403,715
Actual return on plan assets	(28,884)	63,317
Expenses paid	(2,786)	(3,020)
Adjustment	439	0
Benefits paid	(20,925)	(19,923)
Fair value of plan assets at end of period	391,933	444,089
Unfunded status	(139,003)	(123,126)
Accumulated benefit obligation	491,522	520,612
OTHER BENEFITS
Change in benefit obligation
Benefit obligation at beginning of period	43,203	44,136
Service cost	1,320	1,446
Interest cost	1,465	1,569
Plan participants’ contributions	1,224	1,149
Actuarial (gain) loss	(1,106)	437
Expenses paid	0	0
Benefits paid	(5,651)	(5,534)
Benefit obligation at end of period	40,455	43,203
Change in plan assets
Fair value of plan assets at beginning of period	0	0
Actual return on plan assets	0	0
Expenses paid	0	0
Adjustment	0	0
Benefits paid	0	0
Fair value of plan assets at end of period	0	0
Unfunded status	(40,455)	(43,203)
SERP BENEFITS
Change in benefit obligation
Benefit obligation at beginning of period	84,339	78,045
Service cost	542	494
Interest cost	3,077	3,239
Actuarial (gain) loss	(5,163)	6,442
Benefits paid	(4,381)	(4,376)
Plan amendments	0	180
Special/contractual termination benefits	0	315
Benefit obligation at end of period	78,414	84,339
Change in plan assets
Accumulated benefit obligation	$ 78,414	$ 84,339