Derivative Financial Instruments - Summary of Gain (Loss) Recognized in Income on Oil and Gas Commodity Contracts on (Detail) - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Call options [Member] \| Net Realized Investment Gains [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	$ 3.1	$ 1.9

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5618551-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (a),(c),(d),(e)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration