Debt - Additional Information 4 (Detail) (USD $)	1 Months Ended	3 Months Ended	1 Months Ended			3 Months Ended
	Apr. 18, 2013	Mar. 31, 2013	May 01, 2013 Subsequent Event [Member]	Feb. 22, 2013 1.875% Convertible Senior Notes due 2013 [Member]	May 16, 2008 1.875% Convertible Senior Notes due 2013 [Member]	Mar. 31, 2013 1.875% Convertible Senior Notes due 2013 [Member]	Feb. 01, 2013 1.875% Convertible Senior Notes due 2013 [Member]	May 01, 2013 1.875% Convertible Senior Notes due 2013 [Member] Class A common stock [Member] Subsequent Event [Member]
Line of Credit Facility [Line Items]
Convertible senior notes issued					$ 550,000,000
Equity component related to repurchases, convertible, effective interest rate					1.875%	1.875%
Debt instrument, issuance date					May 16, 2008
Interest payable dates						May 1 and November 1
Debt instrument, maturity date	May 01, 2013				May 01, 2013	May 01, 2013
Conversion rate, in shares					24.1196
Common stock principal amount					1,000
Per share conversion price					$ 41.46
Conversion premium					20.00%
Last reported selling price					$ 34.55
Issuance of stock for conversion of debt		5,308,000		18,100,000		5,300,000
Issuance of common stock				437,134		128,332
Initial strike price of warrants						$ 67.37
Exercised share warrents net				71,054		20,616
Settlement of future conversion obligaions							100.00%
Additional conversion notices aggregate value						450,600,000
Converted Notes Held By Subsidiary						81,200,000
Convertble indeture notes						1,764.02
Convertble price per notes						$ 1,000
Converblee notes aggregate value						794,800,000
Aggregate principal amount						142,000
Option settlements				182,900,000		45,200,000
Common stock warrants outstanding								13,156,497
Payment of accrued interest on notes			0
Accrued interest receivable			0
Cash received on hedging transactions		$ 45,300,000