Employee Benefit Plans - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Defined pension plan benefits
Defined Benefit Plan Disclosure [Line Items]
Settlement expense	$ (8,168)	$ 0	$ (8,168)	$ 0
Benefits paid			1,600
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Settlement expense	$ 0	$ 0	0	$ 0
Benefits paid			$ 142
Estimated other postretirement benefits to be paid in 2017					$ 400