Debt - Additional Information 5 (Detail) (USD $)	1 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	1 Months Ended	9 Months Ended					3 Months Ended
	Apr. 24, 2009	Sep. 30, 2013	Sep. 30, 2013 Class A Common Stock [Member]	Sep. 30, 2013 Class A Common Stock [Member]	Apr. 24, 2009 4.0% Convertible Senior Notes due 2014 [Member]	Sep. 30, 2013 4.0% Convertible Senior Notes due 2014 [Member]	Sep. 30, 2013 Convertible Senior Notes [Member]	Dec. 31, 2012 Convertible Senior Notes [Member]	Sep. 30, 2013 Convertible Senior Notes [Member] Maximum [Member]	Sep. 30, 2013 Convertible Senior Notes [Member] Minimum [Member]	Sep. 30, 2013 4.0% Notes [Member]
Line of Credit Facility [Line Items]
Convertible senior notes issued					$ 500,000,000
Debt instrument, issuance date					Apr. 24, 2009
Interest payable dates					April 1 and October 1
Debt instrument, maturity date					Oct. 01, 2014	Oct. 01, 2014
Issuance fee of convertible senior notes	11,700,000
Deferred financing fees	7,700,000
Financing fees recorded as a reduction to shareholders' equity	4,000,000
Initial strike price of warrants						$ 30.38
Equity component related to repurchases, convertible, effective interest rate						4.00%
Conversion rate, in shares						32.9164
Conversion premium						22.50%
Per share conversion price						$ 30.38
Common stock principal amount						1,000
Last reported selling price						$ 24.80
Upper strike price of warrants						$ 44.64
Coupon rate of notes, maximum									12.90%	4.00%
Carrying amount of the equity component							169,000,000	169,000,000
Convertible notes conversion criteria associated with common stock trading price		The last reported sale price of the Company's Class A common stock for at least 20 trading days in the 30 consecutive trading day period ending on the last trading day of the preceding calendar quarter is more than 130% of the applicable conversion price per share of Class A common stock on the last day of such preceding calendar quarter
Convertible notes conversion criteria associated with common stock and convertible notes trading price		Trading price per $1,000 principal amount of the 4.0% Notes for each day in the measurement period was less than 95% of the product of the last reported sale price of Class A common stock and the applicable conversion rate,
Common stock closing price per share			$ 39.49	$ 39.49
Period of payment of additional interest			20 days	20 days
Consecutive trading day			30 days	30 days
Principal amount converted						40,000
Issuance of common stock						1,307
Common stock received						593
Principal amount of conversion notices											$ 3,000