Debt (Details) - Senior Notes	9 Months Ended
Jun. 30, 2016 USD ($)	Apr. 30, 2016 USD ($)
Debt Instrument [Line Items]
Maximum Consolidated Debt to Consolidated Total Capital ratio	0.65
Senior Notes due June 2026
Debt Instrument [Line Items]
Aggregate principal amount	$ 100,000,000	$ 100,000,000
Stated interest rate	2.95%	2.95%
Senior Notes due September 2046
Debt Instrument [Line Items]
Aggregate principal amount		$ 200,000,000
Stated interest rate		4.12%
Senior Notes due October 2046
Debt Instrument [Line Items]
Aggregate principal amount		$ 100,000,000
Stated interest rate		4.12%
Senior Notes due September 2016
Debt Instrument [Line Items]
Aggregate principal amount	$ 175,000,000
Stated interest rate	5.75%

Debt Instrument, Covenant Terms, Maximum Consolidated Debt to Consolidated Total Capital Ratio

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- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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