Marketable Securities Held in Trusts - Maturity Dates and Realized Gain and Loss(Details) - Debt securities $ in Millions	3 Months Ended
Mar. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Gains	$ 0.3
Gross realized losses	(1.9)
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Available-for-sale debt maturities due within one year, fair value	15.5
Available-for-sale debt maturities, after 1 but within 5 years, fair value	374.4
Available-for-sale debt maturities, after 5 but within 10 years, fair value	165.5
Available-for-sale debt maturities, after 10 years, fair value	54.7
Available-for-sale Securities, Debt Securities	$ 610.1

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 320
-Section 50
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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.

+ References

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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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Amount of debt securities categorized neither as held-to-maturity nor trading.

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This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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