Debt (Senior Secured Credit Facility) (Narrative) (Details) (USD $)	0 Months Ended		9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,000,000,000		$ 1,000,000,000	$ 750,000,000
Line of credit facility, maturity date			Oct. 29, 2018
Line of credit facility, letters of credit issued	135,600,000		135,600,000		134,800,000
Line of credit facility, remaining borrowing capacity	262,500,000		262,500,000		562,100,000
Line of credit facility, reduction in remaining borrowing capacity	152,000,000
Line of credit facility, unused facility fee	0.35%	0.30%
Credit Facility [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, amount outstanding	449,500,000	53,000,000	449,500,000		53,000,000
Credit Facility [Member] | Letters of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility, capacity available for letters of credit	262,500,000	315,200,000	262,500,000		315,200,000
Credit Facility [Member] | Letters of Credit [Member] | Performance Standby [Member]
Debt Instrument [Line Items]
Line of credit facility, interest rate	0.875%	0.75%	0.875%		0.75%
Credit Facility [Member] | Letters of Credit [Member] | Financial Standby [Member]
Debt Instrument [Line Items]
Line of credit facility, interest rate	1.75%	1.50%	1.75%		1.50%
Credit Facility [Member] | Plan [Member] | Accordion [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	250,000,000		250,000,000
Credit Facility [Member] | Maximum [Member] | Canadian Dollars and Mexican Pesos [Member]
Debt Instrument [Line Items]
Line of credit facility, revolving loan sublimit	200,000,000		200,000,000
Credit Facility [Member] | Maximum [Member] | Canadian Dollars [Member]
Debt Instrument [Line Items]
Line of credit facility, revolving loan sublimit				$ 100,000,000
Credit Facility [Member] | Weighted Average [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, interest rate	2.12%	2.14%	2.12%		2.14%
Convertible Debt [Member] | Unsecured Debt [Member] | 4.25% Convertible Notes [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	4.25%		4.25%