Interest Rate Swaps (Narrative) (Details) (USD $)	1 Months Ended	6 Months Ended
	Feb. 29, 2012	Mar. 31, 2012	Sep. 30, 2011
Summary of Derivative Instruments by Risk Exposure [Line Items]
Number of interest rate swap agreements		5
Notional amount of interest rate swap agreements		$ 50,000,000	$ 250,000,000
Number of suspended derivative swaps	4
Outstanding credit facility		50,000,000
Interest rate fixed as result of interest rate swap hedges		3.50%
Unrealized loss recognized in OCI		600,000
Unrealized loss recognized on statement of operations		400,000
Interest Rate Swap [Member]
Summary of Derivative Instruments by Risk Exposure [Line Items]
Number of interest rate swap agreements		1
3.4% Interest Rate Swaps Due 2014 [Member]
Summary of Derivative Instruments by Risk Exposure [Line Items]
Number of interest rate swap agreements		4
Outstanding credit facility		$ 250,000,000
Interest rate fixed as result of interest rate swap hedges		3.40%
Maximum [Member]
Summary of Derivative Instruments by Risk Exposure [Line Items]
Derivative swap suspension period, in months	5
Minimum [Member]
Summary of Derivative Instruments by Risk Exposure [Line Items]
Derivative swap suspension period, in months	2