Employee Benefit Plans - Pension Benefits and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended						12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension and Other Postretirement Benefits
Settlement loss	$ 600	$ 1,100	$ 300	$ 2,000	$ 800	$ 500
Pension
Pension and Other Postretirement Benefits
Interest cost							$ 6,705	$ 7,347	$ 7,994
Expected asset return							(10,376)	(10,172)	(12,565)
Amortization of (gain) loss							464	574	186
Net periodic benefit income							(3,207)	(2,251)	(4,385)
Settlement loss							3,995	1,277	8,088
Total benefit (income) expense							$ 788	$ (974)	$ 3,703
Actuarial assumptions:
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets							7.00%	7.00%	7.25%
Expected return on plan assets (as a percent)							7.00%	7.00%	7.25%
Pension | Minimum
Actuarial assumptions:
Discount rate used to measure the plan obligations (as percent)	3.50%				3.81%		3.50%	3.81%
Discount rate (as a percent)							3.56%	3.28%	3.54%
Pension | Maximum
Actuarial assumptions:
Discount rate used to measure the plan obligations (as percent)	3.50%				3.85%		3.50%	3.85%
Discount rate (as a percent)							3.85%	4.10%	4.09%
Other Postretirement Benefits
Pension and Other Postretirement Benefits
Service cost							$ 898	$ 1,104	$ 1,088
Interest cost							2,193	2,637	2,355
Amortization of (gain) loss							(678)	573	588
Total benefit (income) expense							$ 2,413	$ 4,314	$ 4,031
Actuarial assumptions:
Assumed health care cost trend rate, current (as a percent)	7.25%				7.50%		7.25%	7.50%	6.75%
Assumed health care cost trend rate, ultimate (as a percent)	5.00%				5.00%		5.00%	5.00%	5.00%
Other Postretirement Benefits | Minimum
Actuarial assumptions:
Discount rate used to measure the plan obligations (as percent)	3.62%				4.03%		3.62%	4.03%
Discount rate (as a percent)							4.03%	4.32%	3.97%
Other Postretirement Benefits | Maximum
Actuarial assumptions:
Discount rate used to measure the plan obligations (as percent)	3.63%				4.25%		3.63%	4.25%
Discount rate (as a percent)							4.25%	4.56%	4.15%