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INVESTMENTS - Schedule of Investments in Continuous Loss Position (Details) - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017
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Debt Securities, Available-for-sale [Line Items] |  |
Fair value, less than twelve months | $ 354.0 | $ 385.3
Gross unrealized losses, less than twelve months | (4.6) | (1.7)
Fair value, twelve months or greater | 105.0 | 119.5
Gross unrealized losses, twelve months or greater | (1.6) | (1.2)
Fair value, total | 459.0 | 504.8
Gross unrealized losses, total | (6.2) | (2.9)
Commercial paper |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fair value, less than twelve months | 24.6 | 2.4
Gross unrealized losses, less than twelve months | 0.0 | 0.0
Fair value, twelve months or greater | 0.0 | 0.0
Gross unrealized losses, twelve months or greater | 0.0 | 0.0
Fair value, total | 24.6 | 2.4
Gross unrealized losses, total | 0.0 | 0.0
U.S. government and agency securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fair value, less than twelve months | 31.3 | 31.5
Gross unrealized losses, less than twelve months | (0.4) | (0.2)
Fair value, twelve months or greater | 37.0 | 37.1
Gross unrealized losses, twelve months or greater | (0.5) | (0.5)
Fair value, total | 68.3 | 68.6
Gross unrealized losses, total | (0.9) | (0.7)
Foreign government bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fair value, less than twelve months | 3.0 | 3.0
Gross unrealized losses, less than twelve months | 0.0 | 0.0
Fair value, twelve months or greater | 0.0 | 0.0
Gross unrealized losses, twelve months or greater | 0.0 | 0.0
Fair value, total | 3.0 | 3.0
Gross unrealized losses, total | 0.0 | 0.0
Asset-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fair value, less than twelve months | 73.7 | 90.8
Gross unrealized losses, less than twelve months | (0.8) | (0.3)
Fair value, twelve months or greater | 16.1 | 23.2
Gross unrealized losses, twelve months or greater | (0.2) | (0.1)
Fair value, total | 89.8 | 114.0
Gross unrealized losses, total | (1.0) | (0.4)
Corporate debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fair value, less than twelve months | 217.1 | 253.3
Gross unrealized losses, less than twelve months | (3.4) | (1.2)
Fair value, twelve months or greater | 51.9 | 59.2
Gross unrealized losses, twelve months or greater | (0.9) | (0.6)
Fair value, total | 269.0 | 312.5
Gross unrealized losses, total | (4.3) | (1.8)
Municipal securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fair value, less than twelve months | 4.3 | 4.3
Gross unrealized losses, less than twelve months | 0.0 | 0.0
Fair value, twelve months or greater | 0.0 | 0.0
Gross unrealized losses, twelve months or greater | 0.0 | 0.0
Fair value, total | 4.3 | 4.3
Gross unrealized losses, total | $ 0.0 | $ 0.0

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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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