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Equity Items - Additional Information (Details) - USD ($) | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
May 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Accelerated Share Repurchases [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock Repurchase During Period Value | [1] |  |  |  |  |  |  |  |  |  |  |  | $ 200,000,000 |  |
Common shares repurchased |  |  |  |  |  |  |  |  |  |  |  |  | 2,637,054 |  |
Dividend per common share |  |  |  |  |  |  |  |  |  |  |  |  | $ 1.05 | $ 0.95 | $ 1.23
Common Stock, Dividends, Per Share, Declared |  |  | $ 0.275 | $ 0.250 | $ 0.25 | $ 0.225 | $ 0.225 | $ 0.225 | $ 0.225 | $ 0.225 | $ 0.39 | $ 0.39 |  |  |
Common shares reserved (in shares) |  |  | 14,800,000 |  |  |  |  |  |  |  |  |  | 14,800,000 |  |
2018 Stock Repurchase Program [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accelerated Share Repurchases [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock Repurchase Program, Authorized Amount |  |  |  |  |  |  | $ 1,000,000,000 |  |  |  |  |  |  |  |
Stock Repurchase Program, Remaining Authorized Repurchase Amount |  |  | $ 800,000,000 |  |  |  |  |  |  |  |  |  | $ 800,000,000 |  |
2019 Accelerated Share Repurchase Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accelerated Share Repurchases [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock Repurchase During Period Value |  |  |  |  |  |  |  |  |  |  |  |  | $ 200,000,000 |  |
Stock Repurchased And Retired During Period Shares |  | 2,200,000 |  |  |  |  |  |  |  |  |  |  | 2,600,000 |  |
Common shares repurchased |  |  |  |  |  |  |  |  |  |  |  |  | 400,000 |  |
[1] | Common shares repurchased were 2,637,054 for 2019

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