Long-Term Debt (Narrative) (Details) (Unsecured Five-Year Revolving Credit Facility [Member], USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2011
Revolving credit facility, prior borrowing capacity	$ 675.0
Revolving credit facility maturity date	September 22, 2016
Revolving credit facility, restated borrowing capacity	750.0
Revolving credit facility, additional borrowings	250.0
Fees and expenses related to new agreement	2.7
Total spreads and fees	0.975%
Debt instrument, interest rate	1.25%
Borrowings outstanding under the credit facility	$ 120.0
Maximum [Member]
Total spreads and fees	1.40%
Minimum [Member]
Total spreads and fees	0.90%