Debt (Narrative) (Details) (USD $)	3 Months Ended		1 Months Ended			0 Months Ended	1 Months Ended		3 Months Ended	0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Sep. 30, 2014	May 31, 2014	Mar. 27, 2014	Dec. 31, 2012	Oct. 31, 2014	Dec. 31, 2014	May 31, 2014
Debt Instrument [Line Items]
Cash outflow on redemption of debt	$ 2,000,000	$ 3,000,000
Senior Debt Obligations Due 2020 [Member]
Debt Instrument [Line Items]
Proceeds from issuance of debt	500,000,000		500,000,000
Interest rate	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
Interest rate	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
Interest rate	3.50%		3.50%	3.50%
Term of debt				10 years 6 months
Senior Debt Obligations Due 2015 [Member]
Debt Instrument [Line Items]
Interest rate	5.75%		5.75%
Senior Debt Obligations Due 2019 [Member]
Debt Instrument [Line Items]
Interest rate	9.25%		9.25%
Senior Debt Obligations Due 2024 [Member]
Debt Instrument [Line Items]
Proceeds from issuance of debt					600,000,000					600,000,000
Interest rate	3.50%		3.50%		3.50%					3.50%
Term of debt					10 years
Senior Debt Obligations Due 2014 [Member]
Debt Instrument [Line Items]
Interest rate	5.375%		5.375%		5.375%					5.375%
Senior Debt Obligations Due 2023 [Member]
Debt Instrument [Line Items]
Interest rate	4.05%		4.05%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Term of debt						5 years
Line of credit facility, current borrowing capacity						1,000,000,000
Amended Revolving Credit Facility March 27, 2014 [Member]
Debt Instrument [Line Items]
Term of debt						5 years
Debt facilities, maximum borrowing capacity						1,200,000,000
Revolving credit facility, amount outstanding	0		0
Term Loan Facility - due 2016
Debt Instrument [Line Items]
Term of debt							3 years
Debt facilities, maximum borrowing capacity							50,000,000
Revolving credit facility, amount outstanding	50,000,000		50,000,000
Interest rate on three-year draw term loan facility	1.17%		1.17%
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
Senior Debt Obligations Due 2019 [Member]
Debt Instrument [Line Items]
Interest rate								9.25%
Debt partial redemption amount								400,000,000
Senior Debt Obligations Due 2014 [Member]
Debt Instrument [Line Items]
Cash outflow on redemption of debt										320,000,000
Senior Debt Obligations Due 2015 [Member]
Debt Instrument [Line Items]
Interest rate								5.75%
Debt partial redemption amount								$ 230,000,000