Employee Benefit Plans - Pension Benefits and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended						12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension and Other Postretirement Benefits
Settlement loss	$ 800	$ 500	$ 1,700	$ 4,100	$ 1,400	$ 900
Pension
Pension and Other Postretirement Benefits
Interest cost							$ 7,347	$ 7,994	$ 8,844
Expected asset return							(10,172)	(12,565)	(12,664)
Amortization of loss							574	186	135
Net periodic benefit income							(2,251)	(4,385)	(3,685)
Settlement loss							1,277	8,088
Total benefit (income) expense							$ (974)	$ 3,703	$ (3,685)
Actuarial assumptions:
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets							7.00%	7.25%	7.25%
Expected return on plan assets (as a percent)							7.00%	7.25%	7.25%
Pension | Minimum
Actuarial assumptions:
Discount rate used to measure the plan obligations (as percent)	3.81%		4.09%				3.81%	4.09%
Discount rate (as a percent)							3.28%	3.54%	4.40%
Pension | Maximum
Actuarial assumptions:
Discount rate used to measure the plan obligations (as percent)	3.85%		4.10%				3.85%	4.10%
Discount rate (as a percent)							4.10%	4.09%	4.62%
Other Postretirement Benefits
Pension and Other Postretirement Benefits
Service cost							$ 1,104	$ 1,088	$ 897
Interest cost							2,637	2,355	2,384
Amortization of loss							573	588	60
Total benefit (income) expense							$ 4,314	$ 4,031	$ 3,341
Actuarial assumptions:
Assumed health care cost trend rate, current (as a percent)							7.50%	6.75%	7.00%
Assumed health care cost trend rate, ultimate (as a percent)							5.00%	5.00%	5.00%
Other Postretirement Benefits | Minimum
Actuarial assumptions:
Discount rate used to measure the plan obligations (as percent)	4.03%		4.32%				4.03%	4.32%
Discount rate (as a percent)							4.32%	3.97%	4.75%
Other Postretirement Benefits | Maximum
Actuarial assumptions:
Discount rate used to measure the plan obligations (as percent)	4.25%		4.56%				4.25%	4.56%
Discount rate (as a percent)							4.56%	4.15%	5.01%