Employer Sponsored Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Components of net periodic benefit cost
Interest cost	$ 470	$ 514	$ 940	$ 1,029
Expected return on assets	(650)	(594)	(1,300)	(1,188)
Amortization of actuarial loss	256	273	512	546
Net periodic benefit cost	$ 76	$ 193	$ 152	$ 387