Debt (Credit Facility) (Narrative) (Details) (USD $)	0 Months Ended		3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014
Debt Instrument [Line Items]
Line of credit facility, expiration date			Oct. 29, 2018
Line of credit facility, maximum borrowing capacity	$ 750,000,000		$ 750,000,000
Letters of credit outstanding	134,700,000	134,800,000	134,700,000
Line of credit facility, remaining borrowing capacity	486,800,000	562,100,000	486,800,000
Unused credit facility fee	0.30%	0.30%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility, amount outstanding	128,600,000	53,000,000	128,600,000
Revolving Credit Facility [Member] | Weighted Average [Member]
Debt Instrument [Line Items]
Line of credit facility, interest rate	2.08%	2.14%	2.08%
Letter of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility, capacity available for issuance of letters of credit	315,300,000	315,200,000	315,300,000
Letter of Credit [Member] | Performance Standby [Member]
Debt Instrument [Line Items]
Line of credit facility, interest rate	0.75%	0.75%	0.75%
Letter of Credit [Member] | Financial Standby [Member]
Debt Instrument [Line Items]
Line of credit facility, interest rate	1.50%	1.50%	1.50%
Canadian Dollars [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Line of credit facility, revolving loan sublimit	$ 100,000,000		$ 100,000,000