Employee Benefit Plans - Components of Net Periodic Pension/Postretirement Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year				$ 2	$ 3
Interest cost on projected benefit obligations			$ 305	1,228	1,711
Expected return on plan assets			(835)	(1,596)	(2,199)
Amortization of net loss			262	525	575
Settlement expense	$ 11,700	$ 8,200	19,900	641	1,510
Net periodic pension/postretirement benefit cost			19,632	800	1,600
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year			124	121	137
Interest cost on projected benefit obligations			189	195	257
Net gain			(30)	(49)	0
Amortization of prior service credit			(13)	(25)	(25)
Net periodic pension/postretirement benefit cost			$ 270	$ 242	$ 369