Debt - Interest Expense on Debt (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Interest Costs Incurred	$ 47.9us-gaap_InterestCostsIncurred	$ 39.2us-gaap_InterestCostsIncurred	$ 39.8us-gaap_InterestCostsIncurred
Capitalization of Interest	(1.0)	(0.9)	(1.8)
Total Interest Expense	46.9us-gaap_InterestExpenseDebt	38.3us-gaap_InterestExpenseDebt	38.0us-gaap_InterestExpenseDebt
Senior Notes [Member] \| Senior Notes, 6% Due November 30, 2015 [Member]
Debt Instrument [Line Items]
Interest Costs Incurred	15.5	15.4	15.4
Senior Notes [Member] \| Senior Notes, 6% Due May 15, 2017 [Member]
Debt Instrument [Line Items]
Interest Costs Incurred	22.2	22.2	22.1
Subordinated Debt [Member] \| Subordinated Debenture, 7.375 Percent Due 2054 [Member]
Debt Instrument [Line Items]
Interest Costs Incurred	9.4	0	0
Mortgage Note Payable [Member]
Debt Instrument [Line Items]
Interest Costs Incurred	0	0.2	0.4
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Interest Costs Incurred	$ 0.8	$ 1.4	$ 1.9

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Total interest costs incurred during the period and either capitalized or charged against earnings.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391

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- Definition

Amount of interest capitalized, including amount of allowance for funds used during construction.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Allowance for Funds Used during Construction

-URI http://asc.fasb.org/extlink&oid=6504829

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 835

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6501662&loc=d3e56162-110433

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- Definition

Amount of the cost of borrowed funds accounted for as interest expense for debt.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.8)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 8

-Article 5

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