Debt (Details) (USD $)	3 Months Ended					9 Months Ended						9 Months Ended							9 Months Ended			3 Months Ended
	Dec. 31, 2012	Jun. 30, 2013	Sep. 30, 2012	Jun. 30, 2013 Series D, due August 15, 2014, 6.56%	Sep. 30, 2012 Series D, due August 15, 2014, 6.56%	Jun. 30, 2013 Series D, due August 15, 2014, 6.56% Maximum	Jun. 30, 2013 Unsecured senior notes issued July 21, 2009	Jun. 30, 2013 Unsecured senior notes issued July 21, 2009 Maximum	Jun. 30, 2013 Unsecured senior notes issued July 21, 2009 Minimum	Jun. 30, 2013 Due July 21, 2013, 6.10%	Sep. 30, 2012 Due July 21, 2013, 6.10%	Jun. 30, 2013 Due July 21, 2013, 6.10% Subsequent Event	Jun. 30, 2013 Due July 21, 2014, 6.10%	Sep. 30, 2012 Due July 21, 2014, 6.10%	Jun. 30, 2013 Due July 21, 2015, 6.10%	Sep. 30, 2012 Due July 21, 2015, 6.10%	Jun. 30, 2013 Due July 21, 2016, 6.10%	Sep. 30, 2012 Due July 21, 2016, 6.10%	Jun. 30, 2013 Unsecured revolving credit facility mature May 25, 2017	Jun. 30, 2013 Unsecured revolving credit facility mature May 25, 2017 Maximum	Jun. 30, 2013 Unsecured revolving credit facility mature May 25, 2017 Minimum	Dec. 31, 2012 Letter of credit, Replace collateral trusts item	Jun. 30, 2013 Letter of credit, Replace collateral trusts item	Jun. 30, 2013 Letter of credit, Line of credit guarantee item	Jun. 30, 2013 Letter of credit, Performance guarantee item
Debt
Debt outstanding		$ 235,000,000	$ 235,000,000	$ 75,000,000	$ 75,000,000		$ 160,000,000			$ 40,000,000	$ 40,000,000		$ 40,000,000	$ 40,000,000	$ 40,000,000	$ 40,000,000	$ 40,000,000	$ 40,000,000
Less long-term debt due within one year		40,000,000	40,000,000
Long-term debt		195,000,000	195,000,000
Long-term debt stated interest rate percentage				6.56%			6.10%			6.10%			6.10%		6.10%		6.10%
Ratio of debt to total capitalization (as a percent)						55.00%
Annual principal repayments							40,000,000					40,000,000
Funded leverage ratio (as a percent)								55.00%												50.00%
Interest coverage ratio									2.50												3.00
Borrowing amount																			300,000,000
Base rate for the variable rate																			LIBOR
LIBOR spread on borrowings (as a percent)																			1.125%	1.75%	1.125%
Commitment fee (as a percent)																			0.15%	0.35%	0.15%
Borrowing amount outstanding																			0
Number of letters of credit outstanding																							2	3	2
Letters of credit outstanding/issued																								30,700,000	12,000,000
Number of funded collateral trusts																						2
Proceeds from termination of collateral trusts	26,100,000
Available borrowing capacity excluding letters of credit transaction																			$ 269,300,000