Significant Accounting Policies (Details 2) shares in Thousands, $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2015 USD ($) item shares	Dec. 31, 2014 USD ($)
Marketable and Non-Marketable Securities
Marketable securities	$ 657.4
Net unrealized gains (losses) recorded in accumulated other comprehensive income (loss)	109.6	$ 103.9
Carrying value of investments under the cost method	168.7	167.1
Other-than-temporary non-cash charges	$ 0.0
Available for sale securities
Marketable and Non-Marketable Securities
Number of investment in publicly traded REIT accounted for as available-for-sale security \| item	1
Number of shares owned \| shares	5,710
Market value of investments	$ 481.7	476.4
Aggregate unrealized gain (loss) on available-for-sale investments, net	$ 107.8	$ 102.5
Available for sale securities \| Securities in captive insurance subsidiary portfolio \| Minimum
Marketable and Non-Marketable Securities
Investment maturity period	1 year
Available for sale securities \| Securities in captive insurance subsidiary portfolio \| Maximum
Marketable and Non-Marketable Securities
Investment maturity period	10 years

Represents the maturity period of the investments held by the entity.

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Represents the investment in a publicly traded REIT accounted as an available-for-sale security.

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Accumulated appreciation or loss, net of tax, in value of the total of available-for-sale securities at the end of an accounting period.

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

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Amount, after adjustment, of cost-method investment. Adjustments include, but are not limited to, dividends received in excess of earnings after date of investment that are considered a return of investment and other than temporary impairments.

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This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the cost method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale.

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The aggregate net change in the difference between the fair value and the carrying value, or in the comparative fair values, of marketable securities categorized as trading held at each balance sheet date, that was included in earnings for the period, which may have arisen from (a) securities classified as trading, (b) the unrealized holding gain (loss) on held-to-maturity securities transferred to the trading security category, and (c) the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) on available-for-sale securities transferred to trading securities during the period.

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