Stockholders' Equity (Employee Stock Purchase Plan) (Details) (Employee Stock [Member], Class A Common Stock [Member], USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Employee Stock [Member] | Class A Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cash Proceeds	$ 32,861,000	$ 26,813,000	$ 32,861,000	$ 26,813,000
Class A common shares purchased	424	407	424	407
Weighted-average price per share	$ 77.58	$ 65.90	$ 77.58	$ 65.90
Other Employee Related Liabilities, Current	$ 34,600,000		$ 34,600,000