Employee Benefit Plans - Components of Net Periodic Benefit Cost and Other Amounts Recognized in Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Components of Net Periodic Postretirement Benefit Cost
Service cost	$ 1,163	$ 1,743	$ 2,323
Interest cost	810	1,123	$ 1,498
Amortization of prior service cost	(934)	83
Amortization of net actuarial gain	0	0
Net periodic postretirement cost	1,039	2,949
Recognized curtailment loss (one-time cost)	(4,850)	0
Total postretirement cost	(3,811)	2,949
Other Changes in Benefit Obligations Recognized in Other Comprehensive Income (Loss)
Net (gain) loss	2,266	0
Amortization of prior service cost	2,416	(83)
Prior service credit	(8,643)	0
Total recognized in other comprehensive income	(3,961)	(83)
Total recognized in net periodic benefit cost and other comprehensive income (loss)	$ (7,772)	$ 2,866