Acquisitions - Summary of Unaudited Pro Forma Results of Operations (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Business Acquisition [Line Items]
Revenue	$ 656,896	$ 653,727	$ 1,974,229	$ 1,955,452
Net Income	$ 22,096	$ 37,773	$ 92,781	$ 103,387
Class L Common Stock
Business Acquisition [Line Items]
Income (loss) per share		$ 4.81		$ 13.78
Income (loss) per share		$ 4.62		$ 13.22
Common Class A [Member]
Business Acquisition [Line Items]
Income (loss) per share	$ 0.04	$ (0.12)	$ 0.19	$ (0.39)
Income (loss) per share	$ 0.04	$ (0.12)	$ 0.18	$ (0.39)