Fair Value of Financial Instruments (Details) (USD $)	6 Months Ended
Jun. 30, 2011
Debt Instrument [Line Items]
Underlying risk	interest rate risk
Notional amount	$ 500,000,000
Type of instrument	interest rate swaps
Hedge designation	fair value hedges
Amount of hedged item	500,000,000
Description of hedged item	senior notes due 2013
Basis spread on Variable rate	4.377%
Senior Notes due 2019
Debt Instrument [Line Items]
Face Value of Debt Instrument	500,000,000
Fair Value of Debt Instrument	568,000,000
Senior Notes due 2013
Debt Instrument [Line Items]
Face Value of Debt Instrument	500,000,000
Fair Value of Debt Instrument	555,000,000
Fair Value Inputs - Level 2
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Interest Rate Fair Value Hedge Asset	15,000,000
Interest Rate Fair Value Hedge Liability	$ 15,050,000

Total monetary amount specified by the derivative(s). Expressed as an absolute value.

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Period Type:	instant

- Definition

The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Principles Board Opinion (APB)
-Number 21
-Paragraph 16, 20

+ Details

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

- Definition

Estimated fair value of the debt instrument at the balance-sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 107
-Paragraph 10

+ Details

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

- Definition

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.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Amount of the hedged item as of the balance sheet date related to the derivative. For example, the hedged balance on a debt instrument.

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

The average percentage points added to the reference rate to compute the variable rate on the group of interest rate derivatives.

+ Details

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Data Type:	us-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Description of the hedged item related to the derivative. Includes the type of instrument hedged, contract amount, term, and so forth.

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The hedge designation, if any, elected for the derivative.

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of the type of derivative instrument, such as a swap, forward, and option. Excludes the underlying risk of the instrument.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Description of the primary underlying risk exposure associated with the derivative. For example, interest rate risk, credit risk, foreign exchange rate risk, or overall price risk.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

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.

+ Details

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- Definition

Fair value as of the balance sheet date of all derivative assets designated as interest rate fair value hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 133
-Paragraph 17

+ Details

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- Definition

Fair value as of the balance sheet date of all derivative liabilities designated as interest rate fair value hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 133
-Paragraph 17

+ Details

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