Note Net income per common share - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
Dec. 31, 2018	Sep. 30, 2018
Earnings Per Share, Basic and Diluted, Other Disclosures [Abstract]
Accelerated Share Repurchases Final Price Paid Per Share	$ 51.27
Initial shares received as part of an accelerated share repurchase program		2,000,000
Accelerated share repurchases settlement receipt	$ 125
Common Shares Repurchased Accelerated Share Repurchase Agreements	438,180

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- Definition

Initial shares received as part of an accelerated share repurchase program.

+ References

No definition available.

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- Definition

Final price paid per share for the purchase of the targeted number of shares, determined by an average market price over a fixed period of time.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 25
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6405686&loc=d3e22802-112653

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- Definition

Amount of cash receipt from (payment to) bank; or stock received from (issuance to) bank in the settlement of the accelerated share repurchase agreement.

+ References

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- References

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