Derivative Instruments and Hedging Activities Derivative Instruments and Hedging Activities - Fixed Rate Bonds (Details) $ in Millions	Jun. 30, 2016 Swap	Sep. 30, 2015 Swap	Jun. 30, 2015 Swap	Jun. 30, 2014 USD ($) Swap	Sep. 30, 2013 USD ($) Swap
Derivative Instruments and Hedging Activities - Fixed Rate Bonds [Line Items]
Number of Interest Rate Derivatives Held	8	12	12
5.5% due 2016 | Interest Rate Swaps
Derivative Instruments and Hedging Activities - Fixed Rate Bonds [Line Items]
Number of Interest Rate Derivatives Held					4
Notional amount | $					$ 800
Debt Instrument, Interest Rate, Stated Percentage					5.50%
2.6% due 2017 | Interest Rate Swaps
Derivative Instruments and Hedging Activities - Fixed Rate Bonds [Line Items]
Number of Interest Rate Derivatives Held				4
Notional amount | $				$ 400
Debt Instrument, Interest Rate, Stated Percentage				2.60%
1.4% due 2018 | Interest Rate Swaps
Derivative Instruments and Hedging Activities - Fixed Rate Bonds [Line Items]
Number of Interest Rate Derivatives Held				3
Notional amount | $				$ 300
Debt Instrument, Interest Rate, Stated Percentage				1.40%
7.125% due 2017 | Interest Rate Swaps
Derivative Instruments and Hedging Activities - Fixed Rate Bonds [Line Items]
Number of Interest Rate Derivatives Held				1
Notional amount | $				$ 150
Debt Instrument, Interest Rate, Stated Percentage				7.125%