Debt - Credit Facility (Narrative) (Details) (Revolving Credit Facility [Member], USD $) In Millions, unless otherwise specified	0 Months Ended
Sep. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Line of credit facility, remaining borrowing capacity	$ 425.2	$ 345.2
Unused credit facility fee	0.40%	0.35%
Line of Credit [Member]
Debt Instrument [Line Items]
Credit facility, amount outstanding	36.4	134.0
Weighted Average [Member] \| Line of Credit [Member]
Debt Instrument [Line Items]
Line of credit interest rate	3.93%	3.95%
Letter of Credit [Member]
Debt Instrument [Line Items]
Letters of credit issued	138.5	120.8
Line of credit facility, capacity available for issuance of letters of credit	$ 211.5	$ 229.2
Letter of Credit [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Line of credit interest rate	1.125%	1.00%
Letter of Credit [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Line of credit interest rate	2.25%	2.00%

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount borrowed under the credit facility as of the balance sheet date.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The maximum amount of borrowing capacity under a line of credit that is available as of the balance sheet date for a specific purpose other than for financing goods acquired for inventory or imminent delivery to a customer.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityCapacityAvailableForSpecificPurposeOtherThanForTradePurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The effective interest rate at the end of the reporting period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

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- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

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