Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined benefit pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,014	$ 1,198	$ 3,206	$ 3,441
Interest cost	4,494	4,479	13,691	13,370
Expected return on plan assets	(5,621)	(5,768)	(16,952)	(17,253)
Amortization of prior service cost	7	8	23	23
Recognized actuarial (gain) loss	2,125	1,823	6,648	5,464
Settlement loss	0	0	2,443	0
Net periodic benefit cost	2,019	1,740	9,059	5,045
Postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	1	1
Interest cost	52	203	156	609
Amortization of prior service cost	(509)	0	(1,528)	0
Recognized actuarial (gain) loss	(80)	37	(240)	112
Net periodic benefit cost	$ (537)	$ 240	$ (1,611)	$ 722