Pension Plan and Postretirement Benefits Other Than Pension - Components of net periodic costs, weighted average assumptions, and expected benefit payments (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Components of net periodic benefit cost:
Service cost			$ 8,367	$ 12,199
Interest cost		$ 5,986	6,248	9,432
Expected return on plan assets		(8,320)	(10,113)	(13,927)
Actuarial (gain) loss		(15,501)	14,091	(8,891)
Curtailment gain			(31,621)
Settlement loss				20,681
Total		$ (17,835)	$ (13,028)	$ 19,494
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)		3.76%		4.60%
Expected return on plan assets (as a percent)	6.00%	5.00%		7.50%
Rate of compensation increase (as a percent)			5.12%	5.12%
Expected benefit payments
2019		$ 7,984
2020		8,068
2021		9,371
2022		8,843
2023		9,031
2024 through 2028		46,521
Total		$ 89,818
Pension Benefits | Maximum
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)		3.76%	4.39%
Expected return on plan assets (as a percent)			7.00%
Pension Benefits | Minimum
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)			3.96%
Expected return on plan assets (as a percent)			6.00%
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost				$ 555
Interest cost		$ 54	$ 58	297
Actuarial gain amortization		(120)	(180)	(153)
Prior service cost amortization		(2)	(4)	4
Curtailment gain				(8,475)
Total		$ (68)	$ (126)	$ (7,772)
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)		3.28%	3.46%	4.44%
Expected benefit payments
2019		$ 250
2020		179
2021		131
2022		116
2023		79
2024 through 2028		233
Total		988
Contributions by participants		$ 366	$ 506	$ 532