Fair value measurements (Details) (USD $) In Millions	12 Months Ended
Dec. 31, 2010	Dec. 31, 2009
Fair Value Measurements (Textuals)
Restricted cash and investments	$ 16.3	$ 9.1
Average Maturity period of investment in AAA-rated money market mutual funds, days	less than 90 days
Carrying values and the fair values of Senior Notes
Carrying Amount	2,493.6	2,492.3
Fair Value	2,758.3	2,755.9
Fair Value, Inputs, Level 1 [Member]
Fair Value Measurements (Textuals)
Cash equivalents	426.3	909.8
Restricted cash and investments	16.3	9.1
Trading securities, included in other assets	13.5	11.4
5.25% senior notes due 2012, net of unamortized discount
Carrying values and the fair values of Senior Notes
Carrying Amount	999.6	999.4
Fair Value	1,056.0	1,068.6
6.25% senior notes due 2014, net of unamortized discount
Carrying values and the fair values of Senior Notes
Carrying Amount	996.9	996.1
Fair Value	1,116.0	1,095.7
7.25% senior notes due 2019, net of unamortized discount
Carrying values and the fair values of Senior Notes
Carrying Amount	497.1	496.8
Fair Value	$ 586.3	$ 591.6

Average Maturity period of investment in money market mutual fund.

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- Definition

Fair Value Measurements.

+ Details

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- Definition

This element represents a certain statement of financial position asset caption which represents a class of assets, or which may include an individual asset, measured at fair value on a recurring basis.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph a

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+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph a

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- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents notes payable 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, 20, 22

-Article 5

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

+ Details

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The current cash, cash equivalents and investments that are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. Includes current cash equivalents and investments that are similarly restricted as to withdrawal, usage or disposal.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 1

-Article 5

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Carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion. Senior note holders are paid off in full before any payments are made to junior note holders.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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