EMPLOYEE BENEFIT PLANS - Components of Net Periodic Benefit Cost (Details) - Qualified Pension Plan - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Qualified Plan
EMPLOYEE BENEFIT PLANS
Service cost—benefits earned during the period			$ 13,185	$ 12,011	$ 12,791
Interest cost on projected benefit obligation			9,830	9,846	9,751
Expected return on plan assets			(17,058)	(13,570)	(13,780)
Amortization of prior service cost/(credit)			0	0	0
Amortization of actuarial loss/(gain)			0	0	0
Preliminary net periodic benefit cost			5,957	8,287	8,762
Settlement/curtailment expense			0	0	(964)
Total net periodic benefit cost			$ 5,957	8,287	7,798
Average remaining service period used			9 years 2 months 3 days
Nonqualified Plan
EMPLOYEE BENEFIT PLANS
Service cost—benefits earned during the period			$ 1,415	1,350	1,413
Interest cost on projected benefit obligation			1,436	1,480	1,353
Expected return on plan assets			0	0	0
Amortization of prior service cost/(credit)			0	0	0
Amortization of actuarial loss/(gain)			969	634	178
Preliminary net periodic benefit cost			3,820	3,464	2,944
Settlement/curtailment expense			1,526	0	2,135
Total net periodic benefit cost			$ 5,346	$ 3,464	$ 5,079
Average remaining service period used	7 years 10 months 14 days	7 years 8 months 24 days