Debt (Details)	Jul. 16, 2015 USD ($) option subsidiary	Sep. 30, 2015 USD ($)	Jul. 28, 2015 USD ($)	Sep. 30, 2014 USD ($)
Publicly Traded Debt [Member]
Financial Instruments
Carrying Amount		$ 1,918,482,000		$ 1,620,856,000
Fair Value		1,958,636,000		1,651,232,000
Non-traded debt [Member]
Financial Instruments
Carrying Amount		4,851,000		4,121,000
Fair Value		$ 4,620,000		$ 3,914,000
Line of Credit [Member]
Debt Instrument [Line Items]
Number of subsidiaries | subsidiary	3
Term (in years)	5 years
Maximum borrowing capacity	$ 1,350,000,000
Number of extension options | option	2
Term of extension options (in years)	1 year
Maximum borrowing capacity pursuant to extension options	$ 1,850,000,000
Senior Notes Due 2025 [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Face amount			$ 400,000,000
Interest rate (percentage)			3.45%
Senior Notes Due 2045 [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Face amount			$ 400,000,000
Interest rate (percentage)			4.55%