Employee Benefit Plans - Changes in Net Periodic Benefit Cost (Details) - Postretirement Benefits Other than Pensions - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of Net Periodic Postretirement Benefit Cost
Service cost	$ 2,323	$ 1,808	$ 1,295
Interest cost	1,498	1,448	1,343
Amortization of prior service cost	111	0	0
Amortization of net loss	0	0	(26)
Net periodic postretirement cost	3,932	3,256	2,612
Other Changes in Benefit Obligations Recognized in Other Comprehensive Income (Loss)
Net (gain) loss	(2,846)	(2,829)	373
Prior service cost	815	0	0
Amortization of prior service cost	(111)	0	0
Amortization of net loss	0	0	26
Total recognized in other comprehensive income	(2,142)	(2,829)	399
Total recognized in net periodic benefit cost and other comprehensive income (loss)	$ 1,790	$ 427	$ 3,011