Financing (Details Textuals) (USD $)	6 Months Ended
	Jun. 30, 2011	Aug. 13, 2010
Financing (Textuals) [Abstract]
Line of credit facility duration	3 years
Redemption value of senior notes	prior to maturity at a price equal to the greater of (1) 100% of the aggregate principal amount of any notes being redeemed, plus accrued and unpaid interest; or (2) the sum of the present values of the remaining scheduled payments of principal and interest on the notes being redeemed, not including unpaid interest accrued to the redemption date, discounted to the redemption date on a semiannual basis
Senior note redemption discount rate description	At the treasury rate plus 20 basis points
Financing cost related to senior notes	$ 10,900,000
Amortization period of senior notes	5 years
Revolving credit facility		750,000,000
Revolving credit facility due	100,000,000
Maximum [Member]
Line of Credit Facility [Line Items]
LIBOR margin interest rate	1.95%
Commitment fee on the unused portion of the revolving credit facility	0.30%
Minimum [Member]
Line of Credit Facility [Line Items]
LIBOR margin interest rate	1.55%
Commitment fee on the unused portion of the revolving credit facility	0.20%
5.25% senior notes due 2012, net of unamortized discount [Member]
Debt Instrument [Line Items]
Interest rate on debt instruments	5.25%
6.25% senior notes due 2014, net of unamortized discount [Member]
Debt Instrument [Line Items]
Interest rate on debt instruments	6.25%
3.125% senior notes due 2016, net of unamortized discount [Member]
Debt Instrument [Line Items]
Interest rate on debt instruments	3.125%
Senior Notes Aggregate principal amount	$ 1,500,000,000
7.25% senior notes due 2019, net of unamortized discount [Member]
Debt Instrument [Line Items]
Interest rate on debt instruments	7.25%