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Summary of Significant Accounting Policies - Stock-Based Compensation (Details) - $ / shares |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2016 | Sep. 30, 2015 | Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Employee stock options granted (in shares) |  | 0 | 750,000 | 0 |
Shares issued (in dollars per share) | $ 27.15 |  |  |  |
Period of consecutive trading days | 20 days |  |  |  |
Period following an executive's termination of employment for a number of reasons | 18 months |  |  |  |
Restricted Shares |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Restricted shares of common stock awarded (in shares) |  |  | 0 | 0 | 12,500
Weighted average fair value (in dollars per share) |  |  | $ 0.00 | $ 0.00 | $ 49.49
Employee Stock Option |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Employee stock options granted (in shares) |  |  | 750,000 |  |
Weighted average grant date fair value of option grant (in dollars per share) |  |  | $ 3.53 |  |
Chief Executive Officer |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Employee stock options granted (in shares) | 300,000 |  |  |  |
Board of Directors Chairman |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Employee stock options granted (in shares) | 225,000 |  |  |  |
Tranche One |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Percentage of option price to vest | 25.00% |  |  |  |
Amount per share (in dollars per share) | $ 60.00 |  |  |  |
Tranche Two |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Percentage of option price to vest | 25.00% |  |  |  |
Amount per share (in dollars per share) | $ 75.00 |  |  |  |
Tranche Three |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Percentage of option price to vest | 25.00% |  |  |  |
Amount per share (in dollars per share) | $ 90.00 |  |  |  |
Tranche Four |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Percentage of option price to vest | 25.00% |  |  |  |
Amount per share (in dollars per share) | $ 105.00 |  |  |  |

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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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