INCOME TAXES - PROVISION AND DEFERRED TAX (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Effective tax rate that differs from that computed at the federal statutory rate
Federal rate times financial statement income (as a percent)	21.00%		35.00%	21.00%	35.00%	35.00%
Effect of:
State taxes, net of federal benefit (as a percent)	0.82%		1.41%	1.54%	0.64%
General business tax credits (as a percent)	(0.63%)			(0.31%)
Nontaxable income (as a percent)	(1.35%)		(2.54%)	(0.94%)	(1.55%)
Other, net (as a percent)	1.10%		0.17%	(0.11%)	(0.45%)
Effective tax rate (as a percent)	20.94%		34.04%	21.18%	33.64%
Income tax expense as result of reduced tax rate		$ 6.3