INCOME TAXES - PROVISION AND DEFERRED TAX (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2017
Effective tax rate that differs from that computed at the federal statutory rate
Federal statutory rate times financial statement income (as a percent)	21.00%		35.00%	35.00%
Effect of:
State taxes, net of federal benefit (as a percent)	1.95%		0.26%
General business tax credits (as a percent)	(0.41%)
Nontaxable income (as a percent)	(0.71%)		(1.05%)
Other, net (as a percent)	(0.52%)		(0.78%)
Effective tax rate (as a percent)	21.31%		33.43%
Income tax expense as result of reduced tax rate		$ 6.3