Debt (Narrative) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Mar. 26, 2012
Federal Funds Rate [Member]
Debt Instrument [Line Items]
Interest rate (as a percent)	0.50%
One-Month LIBOR [Member]
Debt Instrument [Line Items]
Interest rate (as a percent)	1.00%
Revolving Credit Facility Maturing On March 2017 [Member]
Debt Instrument [Line Items]
Line of credit facility, initiation date	2012-03-26
Maximum capacity for letters of credit under credit facility		$ 1,500.0
Conditional increase in maximum borrowing capacity under line of credit facility	500
Term of debt (in years)	5
Maturity date	Mar. 01, 2017
Five-Year Revolving Agreement [Member]
Debt Instrument [Line Items]
Maximum capacity for letters of credit under credit facility	600.0
Term of debt (in years)	5
Maturity date	Dec. 01, 2012
Three-Year Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Maximum capacity for letters of credit under credit facility	$ 350.0
Term of debt (in years)	3
Minimum [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Interest rate (as a percent)	1.125%
Minimum [Member] | Base Rate Loans [Member]
Debt Instrument [Line Items]
Interest rate (as a percent)	0.125%
Maximum [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Interest rate (as a percent)	1.75%
Maximum [Member] | Base Rate Loans [Member]
Debt Instrument [Line Items]
Interest rate (as a percent)	0.75%