Debt (Details) - USD ($)	1 Months Ended	3 Months Ended			7 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013	Jun. 30, 2009	Mar. 31, 2008	Jun. 30, 2003
Debt Instrument [Line Items]
Write off of Deferred Debt Issuance Cost			$ 2,500,000
Amount outstanding under revolving credit facility	$ 0	$ 0			$ 0	$ 0
Revolving credit facility draw down amount without violating covenant	400,000,000	400,000,000			400,000,000	400,000,000
2.375% Notes Make-Whole Premium		5,000,000
Loss on extinguishment of debt		(5,500,000)		$ 0		$ (5,500,000)	$ 0
Credit ratings						Our long-term credit ratings are Ba2 (Negative Outlook) for corporate family debt, and Ba3 (Negative Outlook) for senior unsecured debt, with Moody's; B+ (Stable Outlook) with S&P; and BB- (Negative Outlook) with Fitch, which are below investment grade.
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility	400,000,000	400,000,000			400,000,000	$ 400,000,000
Debt Instrument, Interest Rate Terms			Borrowings under the 2015 revolving credit facility bear interest, at our option, at a rate per annum equal to LIBOR plus 250 basis points or a floating base rate plus 150 basis points, in each case subject to adjustment based upon a leverage-based pricing grid.
2013 Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility	1,000,000,000	1,000,000,000			1,000,000,000	1,000,000,000
Amount drawn under credit facility	0	0			0	0
Termination penalty on revolving credit facility	$ 0	0			$ 0	$ 0
2013 Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate Terms						The indenture governing the 2013 Notes contains interest rate adjustment provisions depending on the long-term credit ratings assigned to the 2013 Notes with S&P and Moody's. As described in the indenture, the interest rates on the 2013 Notes increase by .25% for each one-notch downgrade below investment grade on each of our long-term credit ratings assigned to the 2013 Notes by S&P or Moody's. These adjustments are limited to a total increase of 2% above the respective interest rates in effect on the date of issuance of the 2013 Notes.
Six Point Five Percent Notes Due March Two Thousand Nineteen [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount									$ 350,000,000
Debt Instrument, Interest Rate, Stated Percentage									6.50%
Five Point Seven Five Percent Notes, Due March Two Thousand Eighteen [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount										$ 250,000,000
Debt Instrument, Interest Rate, Stated Percentage										5.75%
Four Point Two Percent Notes, Due July Two Thousand Eighteen [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount											$ 250,000,000
Debt Instrument, Interest Rate, Stated Percentage											4.20%
Two Point Three Seven Five Percent Notes, Due March Two Thousand Sixteen [Member]
Debt Instrument [Line Items]
Write off of Deferred Debt Issuance Cost		$ 500,000
Debt Instrument, Face Amount								$ 250,000,000
Debt Instrument, Interest Rate, Stated Percentage								2.375%
Prepayment percent		100.00%
Accrued Interest Paid on Extinguishment of 2.375% Notes		$ 3,100,000
Loss on extinguishment of debt		$ (5,500,000)
2013 Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Increase (Decrease)	1.00%				0.75%	1.75%
Four Point Six Zero Percent Notes, Due March Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount								$ 500,000,000
Debt Instrument, Interest Rate, Stated Percentage								4.60%
Five Point Zero Percent Notes, Due March Two Thousand Twenty-Three [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount								$ 500,000,000
Debt Instrument, Interest Rate, Stated Percentage								5.00%
Six Point Nine Five Percent Notes, Due March Two Thousand Forty-Three [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount								$ 250,000,000
Debt Instrument, Interest Rate, Stated Percentage								6.95%