Debt (Details Textual) (USD $)				3 Months Ended		3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Jul. 25, 2011	Mar. 31, 2012 $925 Million Credit Facility [Member]	Aug. 01, 2011 $925 Million Credit Facility [Member]	Mar. 31, 2012 6.75% Senior Notes [Member]	Mar. 31, 2012 3.75% Convertible Senior Notes [Member]
Debt (Textual) [Abstract]
Total credit facility			$ 1,000,000,000		$ 925,000,000
Maturity date for credit facility				August 1, 2015
Maturity date for Notes						Nov. 15, 2014	Oct. 01, 2014
Accrued interest on line of credit facility				LIBOR plus 2.25% or the bank's base rate plus 1.25%, subject to adjustment
Spread rate added to LIBOR				2.25%
Spread rate added to bank's base rate				1.25%
Interest rate of Senior Notes						6.75%	3.75%
Debt (Additional Textual) [Abstract]
Debt discount on Convertible Senior Notes	$ 37,447,000	$ 40,754,000