LONG-TERM DEBT - Schedule of Long Term Debt (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Total principal		$ 720.0	$ 634.5
Securities due within one year		(120.0)	(14.5)
Debt issuance costs		(4.1)	(3.7)
Total long-term debt		$ 595.9	616.3
Securities due within one year	[1]	5.72%
Unsecured senior notes | 2.98% to 7.20%, due 2017 to 2051
Debt Instrument [Line Items]
Total principal		$ 720.0	$ 634.5
2017 Weighted- average Coupon percentage	[1]	4.60%
Unsecured senior notes | 2.98% to 7.20%, due 2017 to 2051 | Minimum
Debt Instrument [Line Items]
Interest rate percentage		2.98%
Unsecured senior notes | 2.98% to 7.20%, due 2017 to 2051 | Maximum
Debt Instrument [Line Items]
Interest rate percentage		7.20%

[1] Represents weighted-average coupon rates for debt outstanding as of December 31, 2017.