Investments Held-to-maturity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Available-for-sale Securities	$ 85.2	$ 154.0
Available-for-sale Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	9.9
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	2.8
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	3.2
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	0.0
Available-for-sale Securities	$ 15.9

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Fair value of available-for-sale debt securities maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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Fair value of available-for-sale debt securities maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing after the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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