Employee Benefit Plans - Changes in Net Periodic Benefit Cost (Details) - Postretirement Benefits Other than Pensions - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Components of Net Periodic Postretirement Benefit Cost
Service cost	$ 1,808	$ 1,295	$ 1,739
Interest cost	1,448	1,343	1,500
Amortization of net loss	0	(26)	641
Net periodic postretirement cost	3,256	2,612	3,880
Other Changes in Benefit Obligations Recognized in Other Comprehensive Income (Loss)
Net gain (loss)	(2,829)	373	(7,618)
Amortization of net (gain) loss	0	26	(641)
Total recognized in other comprehensive income	(2,829)	399	(8,259)
Total recognized in net periodic benefit cost and other comprehensive income (loss)	$ 427	$ 3,011	$ (4,379)