Debt (Narrative) (Details) - USD ($)		1 Months Ended							3 Months Ended	9 Months Ended
	Sep. 30, 2015	Oct. 29, 2015	Sep. 30, 2015	Mar. 31, 2015	Oct. 31, 2014	Sep. 30, 2014	May. 31, 2014	Dec. 31, 2012	Dec. 31, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 08, 2015
Debt Instrument [Line Items]
Interest rate	3.75%		3.75%							3.75%
Cash outflow on redemption of debt										$ 8,000,000	$ 327,000,000
3.75% Senior Debt Obligations Due 2026 [Member]
Debt Instrument [Line Items]
Proceeds from issuance of debt	$ 600,000,000		$ 600,000,000							$ 600,000,000
Interest rate	3.75%		3.75%							3.75%
Term of debt			10 years 6 months
2.35% Senior Debt Obligations Due 2020 [Member]
Debt Instrument [Line Items]
Proceeds from issuance of debt				$ 500,000,000
Interest rate	2.35%		2.35%	2.35%						2.35%
Term of debt				5 years
2.35% Senior Debt Obligations Due 2019 [Member]
Debt Instrument [Line Items]
Proceeds from issuance of debt						$ 300,000,000					$ 300,000,000
Interest rate	2.35%		2.35%			2.35%				2.35%	2.35%
Term of debt						5 years
3.50% Senior Debt Obligations Due 2025 [Member]
Debt Instrument [Line Items]
Proceeds from issuance of debt						$ 500,000,000					$ 500,000,000
Interest rate	3.50%		3.50%			3.50%				3.50%	3.50%
Term of debt						10 years 6 months
3.50% Senior Debt Obligations Due 2024 [Member]
Debt Instrument [Line Items]
Proceeds from issuance of debt							$ 600,000,000
Interest rate	3.50%		3.50%				3.50%			3.50%
Term of debt							10 years
5.375% Senior Debt Obligations Due 2014 [Member]
Debt Instrument [Line Items]
Interest rate							5.375%
Amended Revolving Credit Facility March 27, 2014 [Member]
Debt Instrument [Line Items]
Term of debt	5 years
Revolving credit facility, amount outstanding	$ 0		$ 0							$ 0
Debt facilities, maximum borrowing capacity	1,200,000,000		1,200,000,000							1,200,000,000
Term Loan Facility - due 2016
Debt Instrument [Line Items]
Term of debt								3 years
Revolving credit facility, amount outstanding	$ 50,000,000		$ 50,000,000							$ 50,000,000
Debt facilities, maximum borrowing capacity								$ 50,000,000
Interest rate on three-year draw term loan facility	1.20%		1.20%							1.20%
Senior Debt Obligations Due 2015 and Senior Debt Obligations Due 2019 [Member]
Debt Instrument [Line Items]
Debt partial redemption amount					$ 630,000,000
Cash outflow on redemption of debt					$ 765,000,000
Cost for early redemption									$ 137,000,000
9.25% Senior Debt Obligations Due 2019 [Member]
Debt Instrument [Line Items]
Interest rate					9.25%
Debt partial redemption amount					$ 400,000,000
5.375% Senior Debt Obligations Due 2014 [Member]
Debt Instrument [Line Items]
Cash outflow on redemption of debt							$ 320,000,000
5.75% Senior Debt Obligations Due 2015 [Member]
Debt Instrument [Line Items]
Interest rate					5.75%
Debt partial redemption amount					$ 230,000,000
Letter of Credit [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Proceeds from issuance of debt												$ 435,000,000
Subsequent Event [Member]
Debt Instrument [Line Items]
Cash moved into escrow		$ 435,000,000
Line of Credit [Member] | Uncommitted Bank Credit Line [Member]
Debt Instrument [Line Items]
Debt facilities, maximum borrowing capacity	$ 150,000,000		$ 150,000,000							$ 150,000,000
Borrowings under facility	$ 0		$ 0							$ 0