Financing Arrangements (Narrative) (Details) - USD ($)		1 Months Ended		12 Months Ended
	Nov. 17, 2017	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Aug. 31, 2017	Aug. 31, 2016	Jul. 31, 2016
Line of Credit Facility [Line Items]
Repayments of debt				$ 550,000,000	$ 600,000,000	$ 450,000,000
Capital Lease Duration				25 years
Capital lease payment duration				15 years
Capital lease expiration date				Mar. 01, 2029
Capital lease obligations, current		$ 9,900,000		$ 9,900,000
Capital lease obligations, noncurrent		$ 134,000,000		$ 134,000,000
5.75% Notes, Due August 15, 2025 [Member]
Line of Credit Facility [Line Items]
Interest rate							5.75%
Face amount of notes							$ 550,000,000
Maturity date				Aug. 15, 2025
Transaction costs							$ 8,400,000
3.50% notes, due December 2017 [Member]
Line of Credit Facility [Line Items]
Interest rate		3.50%	3.50%	3.50%	3.50%
Repayments of debt			$ 550,000,000
6.875 Notes, Due August 15, 2024 [Member]
Line of Credit Facility [Line Items]
Interest rate								6.875%
Face amount of notes								$ 550,000,000
Maturity date				Aug. 15, 2024
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Amount outstanding		$ 90,700,000		$ 90,700,000
2016 Senior Unsecured Guaranteed Credity Facility [Member]
Line of Credit Facility [Line Items]
Credit facility maximum borrowing capacity		1,100,000,000		$ 1,100,000,000				1,200,000,000
Credit facility, expiration date				Aug. 17, 2021
Amount outstanding		$ 0		$ 0
Leverage ratio	375.00%	400.00%
Interest rate				4.875%
Transaction costs								14,000,000
2011 Unscured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility maximum borrowing capacity								$ 630,000,000	$ 2,000,000,000