EMPLOYEE BENEFIT PLANS - Change in Projected Benefit Obligation, Fair Value of Plan Assets, Funded Status, and Amounts Not Yet Recognized (Details) - USD ($) $ in Thousands	11 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Group life insurance plan
Change in projected benefit obligation:
Projected benefit obligation at beginning of year		$ 9,634	$ 9,063
Service cost		122	102
Interest cost		354	338
Actuarial (gain)/loss		1,347	604
Projected benefit obligation at end of year	$ 9,063	10,978	9,634
Change in plan assets:
Benefits paid		(479)	(473)
Qualified Plan | Qualified Pension Plan
EMPLOYEE BENEFIT PLANS
Accumulated benefit obligation, end of year		278,084	247,595
Change in projected benefit obligation:
Projected benefit obligation at beginning of year		265,848	268,221
Service cost	11,220	12,011	12,791
Interest cost	9,072	9,846	9,751
Amendments		0	0
Actuarial (gain)/loss		26,539	5,988
Benefits paid		(13,821)	(30,903)
Projected benefit obligation at end of year	268,221	300,423	265,848
Change in plan assets:
Fair value of plan assets at beginning of year		201,843	196,042
Actual return on plan assets		29,404	15,815
Employer contributions		43,500	20,889
Benefits paid		(13,821)	(30,903)
Fair value of plan assets at end of year	196,042	260,926	201,843
After reflecting FASB guidance:
Funded status		(39,497)	(64,005)
Amounts recognized in the balance sheet:
Other liabilities		(39,497)	(64,005)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain)/loss		2,850	(7,855)
Prior service cost/(credit)		0	0
Total amounts recognized in AOCI		2,850	(7,855)
Nonqualified Plan | Qualified Pension Plan
EMPLOYEE BENEFIT PLANS
Accumulated benefit obligation, end of year		50,149	45,594
Change in projected benefit obligation:
Projected benefit obligation at beginning of year		47,802	56,985
Service cost	1,229	1,350	1,413
Interest cost	1,499	1,480	1,353
Amendments		0	0
Actuarial (gain)/loss		7,861	4,124
Benefits paid		(3,903)	(16,073)
Projected benefit obligation at end of year	56,985	54,590	47,802
Change in plan assets:
Fair value of plan assets at beginning of year		0	0
Actual return on plan assets		0	0
Employer contributions		3,903	16,073
Benefits paid		(3,903)	(16,073)
Fair value of plan assets at end of year	$ 0	0	0
After reflecting FASB guidance:
Funded status		(54,590)	(47,802)
Amounts recognized in the balance sheet:
Other liabilities		(54,590)	(47,802)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain)/loss		13,521	6,294
Prior service cost/(credit)		0	0
Total amounts recognized in AOCI		$ 13,521	$ 6,294