Investments - Unrealized Gains and Gross Unrealized Losses in Investment in Equity Securities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	$ 561.5	$ 530.0
Gross Unrealized Gains	75.7	74.7
Gross Unrealized Losses	(5.0)	(6.2)
Equity Securities at Fair Value	632.2	598.5
Exchange Traded Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	195.2	122.0
Gross Unrealized Gains	8.2	3.9
Gross Unrealized Losses	(0.7)	(1.0)
Equity Securities at Fair Value	202.7	124.9
Limited Liability Companies and Limited Partnerships [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	160.3	150.3
Gross Unrealized Gains	27.7	25.2
Gross Unrealized Losses	(1.8)	(1.6)
Equity Securities at Fair Value	186.2	173.9
Finance, Insurance and Real Estate [Member] \| Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	85.5	85.4
Gross Unrealized Gains	5.2	2.9
Gross Unrealized Losses	(1.0)	(2.5)
Equity Securities at Fair Value	89.7	85.8
All Industries Other than Finance, Insurance and Real Estate [Member] \| Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	16.3	20.1
Gross Unrealized Gains	3.5	4.4
Gross Unrealized Losses	0	(0.1)
Equity Securities at Fair Value	19.8	24.4
2000 to 3999 Manufacturing [Member] \| Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	43.4	83.4
Gross Unrealized Gains	14.9	21.3
Gross Unrealized Losses	(1.1)	(0.1)
Equity Securities at Fair Value	57.2	104.6
All Industries Other than Manufacturing [Member] \| Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	60.8	68.8
Gross Unrealized Gains	16.2	17.0
Gross Unrealized Losses	(0.4)	(0.9)
Equity Securities at Fair Value	$ 76.6	$ 84.9

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

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This item represents the cost of equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments for other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

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