Pension Plan and Employee Benefits - Components of Periodic Benefit Costs and Weighted-Average Assumptions (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 29, 2017
Weighted-average assumptions used to determine the benefit obligation
Rate of compensation increase				2.81%	2.93%
Weighted-average assumptions used to determine the net benefit cost
Rate of compensation increase				2.81%	2.93%	2.98%
PENSION BENEFITS
Components of periodic benefit costs
Service cost				$ 8,414	$ 9,507	$ 9,039
Interest cost				22,485	20,860	21,648
Expected return on plan assets				(26,502)	(23,773)	(24,064)
Amortizations
Prior service credit				(71)	(71)	(71)
Net loss (gain)				7,849	12,312	10,008
Net periodic benefit cost				$ 12,175	$ 18,835	$ 16,560
Weighted-average assumptions used to determine the benefit obligation
Discount rate				3.43%	4.35%
Weighted-average assumptions used to determine the net benefit cost
Discount rate				4.35%	3.73%	4.27%
Expected return on plan assets				6.55%	5.86%	6.08%
OTHER BENEFITS
Components of periodic benefit costs
Service cost				$ 1,191	$ 1,320	$ 1,446
Interest cost				1,646	1,465	1,569
Expected return on plan assets				0	0	0
Amortizations
Prior service credit				0	0	0
Net loss (gain)				21	135	(50)
Net periodic benefit cost				$ 2,858	$ 2,920	$ 2,965
Weighted-average assumptions used to determine the benefit obligation
Discount rate				3.25%	4.16%
Weighted-average assumptions used to determine the net benefit cost
Discount rate				4.16%	3.47%	3.81%
SERP BENEFITS
Components of periodic benefit costs
Service cost				$ 330	$ 542	$ 494
Interest cost				3,326	3,077	3,239
Amortizations
Prior service credit				(160)	(160)	(190)
Net loss (gain)				1,544	2,913	2,105
Net periodic benefit cost				5,040	6,372	5,648
Special/contractual termination benefits	$ 300			0	0	315
Net periodic benefit cost				$ 5,040	$ 6,372	$ 5,963
Weighted-average assumptions used to determine the benefit obligation
Discount rate	4.08%			3.37%	4.34%		4.22%
Rate of compensation increase				5.00%	5.00%
Weighted-average assumptions used to determine the net benefit cost
Discount rate		4.22%	4.08%	4.34%	3.70%
Rate of compensation increase		5.00%	5.00%	5.00%	5.00%