EMPLOYEE BENEFIT PLANS - Components of Net Periodic Benefit Cost (Details) - Qualified Pension Plan - USD ($) $ in Thousands	1 Months Ended	11 Months Ended	12 Months Ended
	Jan. 31, 2015	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Qualified Plan
EMPLOYEE BENEFIT PLANS
Service cost—benefits earned during the period		$ 11,220	$ 12,011	$ 12,791
Interest cost on projected benefit obligation		9,072	9,846	9,751
Expected return on plan assets		(13,214)	(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		7,078	8,287	8,762
Settlement/curtailment expense		0	0	(964)
Total net periodic benefit cost		7,078	$ 8,287	7,798
Average remaining service period used			9 years 2 months 27 days
Qualified Plan | Predecessor
EMPLOYEE BENEFIT PLANS
Service cost—benefits earned during the period	$ 974
Interest cost on projected benefit obligation	1,002
Expected return on plan assets	(1,293)
Amortization of prior service cost/(credit)	(33)
Amortization of actuarial loss/(gain)	668
Preliminary net periodic benefit cost	1,318
Settlement/curtailment expense	0
Total net periodic benefit cost	1,318
Nonqualified Plan
EMPLOYEE BENEFIT PLANS
Service cost—benefits earned during the period		1,229	$ 1,350	1,413
Interest cost on projected benefit obligation		1,499	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)		0	634	178
Preliminary net periodic benefit cost		2,728	3,464	2,944
Settlement/curtailment expense		0	0	2,135
Total net periodic benefit cost		$ 2,728	$ 3,464	$ 5,079
Average remaining service period used			8 years 2 months 23 days
Nonqualified Plan | Predecessor
EMPLOYEE BENEFIT PLANS
Service cost—benefits earned during the period	95
Interest cost on projected benefit obligation	140
Expected return on plan assets	0
Amortization of prior service cost/(credit)	1
Amortization of actuarial loss/(gain)	138
Preliminary net periodic benefit cost	374
Settlement/curtailment expense	0
Total net periodic benefit cost	$ 374