Long-Term Debt (Narrative) (Details) (USD $)	3 Months Ended			3 Months Ended									1 Months Ended
	Dec. 31, 2011	Oct. 31, 2011	Sep. 30, 2011	Dec. 31, 2011 Leverage Ratio [Member]	Dec. 31, 2011 Debt To Capitalization Ratio [Member]	Dec. 31, 2011 Interest Expense Coverage Ratio [Member]	Dec. 31, 2011 2011 Credit Facility [Member]	Sep. 30, 2011 2011 Credit Facility [Member]	Dec. 31, 2011 2011 Credit Facility [Member] Maximum [Member]	May 31, 2011 2011 Credit Facility [Member] Maximum [Member]	May 31, 2011 2011 Credit Facility [Member] Minimum [Member]	Dec. 31, 2011 Interest Rate Swap [Member]	Oct. 31, 2011 3.4% Interest Rate Swaps Due 2014 [Member]
Line of Credit Facility [Line Items]
Credit facility capacity										$ 1,100,000,000	$ 750,000,000
Incremental commitment									350,000,000
2011 Credit Agreement, description							Borrowings under the credit facility bear interest at the Eurodollar rate plus a margin of 2.50% (approximately 2.8% per annum at December 31, 2011).
2011 Credit Agreement, commitment fee (per annum)							1.00%
Outstanding borrowings	520,000,000
Various financial covenants				ratio of 4.0 to 1.0	ratio of 0.5 to 1.0	ratio of 3.0 to 1.0
Minimum collateral percentage							150.00%
Number of interest rate swap agreements		5	4				5
Notional amount of interest rate swap agreements			50,000,000									50,000,000
Interest rate fixed as result of interest rate swap hedges	3.40%												3.40%
Outstanding credit facility	$ 250,000,000		$ 200,000,000				$ 520,000,000	$ 520,000,000