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	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 losses		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 losses		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
Expected amortization, next fiscal year	$ 600	$ 600