Employee Benefit Plans - Schedule of Change in Benefit Obligation and Plan Assets (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2016
Pension
Change in benefit obligation:
Benefit obligation, beginning balance		$ 229,156	$ 216,992
Service cost	$ 1,780	7,805	7,496	$ 7,621	$ 6,188
Interest cost	2,128	8,489	8,461	8,444	8,096
Actuarial (gain) loss		(30,703)	4,543
Benefits paid		(5,542)	(8,336)
Benefit obligation, ending balance	229,156	209,205	229,156
Change in plan assets:
Fair value of plan assets, beginning balance		289,390	257,513
Actuarial return on plan assets		(10,874)	27,213
Employer contributions		0	13,000
Benefits paid		(5,542)	(8,336)
Fair value of plan assets, ending balance	289,390	272,974	289,390
Funded status at end of year	60,234	63,769	60,234
RIM
Change in benefit obligation:
Benefit obligation, beginning balance		12,243	11,032
Service cost	60	282	238	237	140
Interest cost	111	443	433	429	385
Actuarial (gain) loss		(1,355)	874
Benefits paid		(328)	(334)
Benefit obligation, ending balance	12,243	11,285	12,243
Change in plan assets:
Fair value of plan assets, beginning balance		0	0
Actuarial return on plan assets		0	0
Employer contributions		328	334
Benefits paid		(328)	(334)
Fair value of plan assets, ending balance	0	0	0
Funded status at end of year	(12,243)	(11,285)	(12,243)
Post-retirement
Change in benefit obligation:
Benefit obligation, beginning balance		22,078	22,133
Service cost	92	417	445	471	447
Interest cost	205	796	762	$ 742	$ 854
Actuarial (gain) loss		(1,845)	(726)
Benefits paid		(482)	(536)
Benefit obligation, ending balance	22,078	20,964	22,078
Change in plan assets:
Fair value of plan assets, beginning balance		0	0
Actuarial return on plan assets		0	0
Employer contributions		482	536
Benefits paid		(482)	(536)
Fair value of plan assets, ending balance	0	0	0
Funded status at end of year	$ (22,078)	$ (20,964)	$ (22,078)