Stockholders' Equity (Details) (USD $) In Millions, except Share data	3 Months Ended	6 Months Ended
Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Class of Stock [Line Items]
Value of shares authorized to be repurchased		$ 150.0
Stock repurchase program, remaining authorized repurchase amount		125.0
Common Class A [Member]
Class of Stock [Line Items]
Stock repurchased during period, shares	3,100,000	900,000	3,100,000
Stock repurchased during period, value	$ 110.8	$ 25.0	$ 110.8
Common Class B [Member]
Class of Stock [Line Items]
Stock converted			580,730

The number of shares converted in a noncash (or part noncash) transaction. Noncash is defined as transactions during a period that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

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Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

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- Definition

Equity impact of the value of stock that has been repurchased during the period and has not been retired and is not held in treasury. Some state laws may mandate the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

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