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, 2019	Dec. 31, 2018	Sep. 30, 2017
Pension
Change in benefit obligation:
Benefit obligation, beginning balance		$ 209,205	$ 229,156
Service cost	$ 1,780	6,494	7,805	$ 7,621
Interest cost	2,128	8,569	8,489	8,444
Actuarial (gain) loss		46,001	(30,703)
Benefits paid		(6,443)	(5,542)
Benefit obligation, ending balance	229,156	263,826	209,205
Change in plan assets:
Fair value of plan assets, beginning balance		272,974	289,390
Actuarial return on plan assets		54,816	(10,874)
Employer contributions		35,000	0
Benefits paid		(6,443)	(5,542)
Fair value of plan assets, ending balance	289,390	356,347	272,974
Funded status at end of year		92,521	63,769
RIM
Change in benefit obligation:
Benefit obligation, beginning balance		11,285	12,243
Service cost	60	210	282	237
Interest cost	111	466	443	429
Actuarial (gain) loss		2,074	(1,355)
Benefits paid		(339)	(328)
Benefit obligation, ending balance	12,243	13,696	11,285
Change in plan assets:
Fair value of plan assets, beginning balance		0	0
Actuarial return on plan assets		0	0
Employer contributions		339	328
Benefits paid		(339)	(328)
Fair value of plan assets, ending balance	0	0	0
Funded status at end of year		(13,696)	(11,285)
Post-retirement
Change in benefit obligation:
Benefit obligation, beginning balance		20,964	22,078
Service cost	92	339	417	471
Interest cost	205	826	796	$ 742
Actuarial (gain) loss		3,141	(1,845)
Benefits paid		(667)	(482)
Benefit obligation, ending balance	22,078	24,603	20,964
Change in plan assets:
Fair value of plan assets, beginning balance		0	0
Actuarial return on plan assets		0	0
Employer contributions		667	482
Benefits paid		(667)	(482)
Fair value of plan assets, ending balance	$ 0	0	0
Funded status at end of year		$ (24,603)	$ (20,964)