Employee Benefit Plans - Schedule of Components of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2016
Pension
Defined Benefit Plan Disclosure [Line Items]
Unrecognized prior service costs	$ 0	$ 0	$ 0	$ 0
Unrecognized net actuarial income	59,579	61,731	55,438	74,768
Total accumulated other comprehensive income	59,579	61,731	55,438	74,768
RIM
Defined Benefit Plan Disclosure [Line Items]
Unrecognized prior service costs	0	0	0	0
Unrecognized net actuarial income	3,748	5,515	4,725	5,154
Total accumulated other comprehensive income	3,748	5,515	4,725	5,154
Post-retirement
Defined Benefit Plan Disclosure [Line Items]
Unrecognized prior service costs	0	(106)	(140)	(276)
Unrecognized net actuarial income	4,226	6,395	4,611	8,374
Total accumulated other comprehensive income	$ 4,226	$ 6,289	$ 4,471	$ 8,098