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Restricted Securities Available for Sale (Schedule of Restricted Securities Available for Sale by Aging Catagory) (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
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Restricted Securities Available For Sale [Line Items] |  |
Estimated Fair Value, Less than 12 Months | $ 42.4 | $ 28.2
Gross Unrealized Losses, Less than 12 Months | (0.6) | (0.2)
Estimated Fair Value, 12 Months or More | 11.4 | 12.0
Gross Unrealized Losses, 12 Months or More | (0.2) | (0.1)
Total Estimated Fair Value | 53.8 | 40.2
Total Gross Unrealized Losses | (0.8) | (0.3)
Corporate Bond Securities [Member] |  |
Restricted Securities Available For Sale [Line Items] |  |
Estimated Fair Value, Less than 12 Months | 19.6 | 11.1
Gross Unrealized Losses, Less than 12 Months | (0.3) | (0.1)
Estimated Fair Value, 12 Months or More | 1.2 | 1.9
Gross Unrealized Losses, 12 Months or More | 0.0 | 0.0
Total Estimated Fair Value | 20.8 | 13.0
Total Gross Unrealized Losses | (0.3) | (0.1)
US Government and Agency Securities [Member] |  |
Restricted Securities Available For Sale [Line Items] |  |
Estimated Fair Value, Less than 12 Months | 14.6 | 11.0
Gross Unrealized Losses, Less than 12 Months | (0.2) | (0.1)
Estimated Fair Value, 12 Months or More | 7.0 | 7.9
Gross Unrealized Losses, 12 Months or More | (0.2) | (0.1)
Total Estimated Fair Value | 21.6 | 18.9
Total Gross Unrealized Losses | (0.4) | (0.2)
Asset-backed Securities [Member] |  |
Restricted Securities Available For Sale [Line Items] |  |
Estimated Fair Value, Less than 12 Months | 7.0 | 4.9
Gross Unrealized Losses, Less than 12 Months | (0.1) |
Estimated Fair Value, 12 Months or More | 2.1 | 1.0
Gross Unrealized Losses, 12 Months or More | 0.0 | 0.0
Total Estimated Fair Value | 9.1 | 5.9
Total Gross Unrealized Losses | (0.1) |
Mortgage-backed Securities [Member] |  |
Restricted Securities Available For Sale [Line Items] |  |
Estimated Fair Value, Less than 12 Months | 0.3 | 1.2
Estimated Fair Value, 12 Months or More | 1.1 | 1.2
Gross Unrealized Losses, 12 Months or More | 0.0 | 0.0
Total Estimated Fair Value | 1.4 | $ 2.4
Commercial Paper [Member] |  |
Restricted Securities Available For Sale [Line Items] |  |
Estimated Fair Value, Less than 12 Months | 0.9 |
Estimated Fair Value, 12 Months or More | 0.0 |
Gross Unrealized Losses, 12 Months or More | 0.0 |
Total Estimated Fair Value | $ 0.9 |

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