Long-Term Debt (Summary of Long-Term Debt) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Sep. 30, 2012
Line of Credit Facility [Line Items]
Long-term debt	$ 1,070,000		$ 830,000
Senior Notes [Member] | 6.5% Senior Notes [Member]
Line of Credit Facility [Line Items]
Principal amount	450,000		450,000
Interest rate	6.50%
Line of Credit [Member] | 2011 Credit Facility [Member]
Line of Credit Facility [Line Items]
Outstanding credit facility	$ 620,000		$ 380,000
Market adjustable interest rate (per annum)	3.00%	[1]	3.20%	[1]

[1] After the impact of our interest rate swaps.