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Stockholders' Equity - Narrative (Details) |  |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |  |
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Oct. 25, 2018 $ / shares shares | Oct. 03, 2018 USD ($) $ / shares shares | Sep. 07, 2018 $ / shares shares | Apr. 27, 2018 USD ($) | Apr. 16, 2018 USD ($) | Dec. 31, 2016 $ / shares shares | Nov. 30, 2017 $ / shares shares | Dec. 31, 2018 USD ($) $ / shares shares | Dec. 31, 2017 USD ($) | Sep. 30, 2018 USD ($) $ / shares | Sep. 30, 2016 $ / shares | Sep. 30, 2015 executive $ / shares shares | Sep. 30, 2017 USD ($) shares | May 31, 2017 shares | Mar. 31, 2017 shares
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares issued (in dollars per share) |  | $ 34.00 |  |  |  |  |  |  |  |  |  |  |  |  |
Shares of common stock authorized to be repurchased (up to) (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  | 7,000,000 | 2,000,000 | 5,000,000
Shares issued (in shares) | shares |  | 752,234 |  |  |  |  |  |  |  |  |  |  |  |  |
Cost of shares | $ |  | $ 25,575,956 |  |  |  |  |  |  |  |  |  |  |  |  |
Cash capital contribution required, due to catastrophe | $ |  |  |  |  | $ 2,041,000 |  |  |  |  |  |  |  |  |  |
Capital contribution received from parent | $ |  |  |  | $ 1,041,000 |  |  |  |  |  |  |  |  |  |  |
Capital contribution received from noncontrolling interest | $ |  |  |  |  |  |  |  | $ 1,000,000 |  |  |  |  |  |  |
Corporate, General and Administrative Expense |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense | $ |  |  |  |  |  |  |  | 238,000 | $ 192,000 |  |  |  |  |  |
Restricted Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense | $ |  |  |  |  |  |  |  | 26,000 | 26,000 |  |  |  |  |  |
Restricted shares awarded (in shares) | shares |  |  |  |  |  |  | 5,000 |  |  |  |  | 12,500 |  |  |
Number of senior executives | executive |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |
Weighted average fair value (in dollars per share) |  |  |  |  |  |  | $ 31.95 |  |  |  |  | $ 49.49 |  |  |
Award vesting period |  |  |  |  |  |  | 3 years |  |  |  |  |  |  |  |
Unrecognized expense | $ |  |  |  |  |  |  |  | 146,000 |  |  |  |  | $ 172,000 |  |
Employee Stock Option |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense | $ |  |  |  |  |  |  |  | 289,000 | $ 205,000 |  |  |  |  |  |
Unrecognized expense | $ |  |  |  |  |  |  |  | $ 2,553,000 |  | $ 2,842,000 |  |  |  |  |
Weighted average remaining contractual term |  |  |  |  |  |  |  | 1 year 11 months 23 days |  |  |  |  |  |  |
2017 Authorization Program |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized amount | $ |  |  |  |  |  |  |  | $ 1,676,443 |  |  |  |  |  |  |
Minimum | Restricted Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  |  |  |  |  |  |  |  | 3 years |  |  |
Maximum | Restricted Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  |  |  |  |  |  |  |  | 5 years |  |  |
2019 Option Grants |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares issued (in dollars per share) | $ 33.34 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period following an executive's termination of employment for a number of reasons | 18 months |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period following an executive's termination of employment without cause | 12 months |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share price (in dollars per share) | $ 29.50 |  |  |  |  |  |  |  |  |  |  |  |  |  |
2019 Option Grants | Mr. Kiernan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options granted in period (in shares) | shares | 10,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
2019 Option Grants | Tranche One |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of awards vesting (in shares) | shares | 3,333 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount per share (in dollars per share) | $ 40.00 |  |  |  |  |  |  |  |  |  |  |  |  |  |
2019 Option Grants | Tranche Two |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of awards vesting (in shares) | shares | 3,333 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount per share (in dollars per share) | $ 45.00 |  |  |  |  |  |  |  |  |  |  |  |  |  |
2019 Option Grants | Tranche Three |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of awards vesting (in shares) | shares | 3,334 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount per share (in dollars per share) | $ 50.00 |  |  |  |  |  |  |  |  |  |  |  |  |  |
2019 Option Grants | Employee Stock Option |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Grants in period, weighted average (in dollars per share) |  |  |  |  |  |  |  | $ 7.10 |  |  |  |  |  |  |
2019 Option Grants | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share price (in dollars per share) | $ 40.00 |  |  |  |  |  |  |  |  |  |  |  |  |  |
2018 Option Grants |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares issued (in dollars per share) |  |  | $ 33.60 |  |  |  |  |  |  |  |  |  |  |  |
Period following an executive's termination of employment for a number of reasons |  |  | 18 months |  |  |  |  |  |  |  |  |  |  |  |
Period following an executive's termination of employment without cause |  |  | 12 months |  |  |  |  |  |  |  |  |  |  |  |
Share price (in dollars per share) |  |  |  |  |  |  |  | 29.50 |  |  |  |  |  |  |
2018 Option Grants | Remy W. Trafelet |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options granted in period (in shares) | shares |  |  | 210,000 |  |  |  |  |  |  |  |  |  |  |  |
2018 Option Grants | Mr. Kiernan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options granted in period (in shares) | shares |  |  | 90,000 |  |  |  |  |  |  |  |  |  |  |  |
2018 Option Grants | Tranche One |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount per share (in dollars per share) |  |  | $ 35.00 |  |  |  |  |  |  |  |  |  |  |  |
Percentage of options |  |  | 25.00% |  |  |  |  |  |  |  |  |  |  |  |
Period of consecutive trading days |  |  | 20 years |  |  |  |  |  |  |  |  |  |  |  |
2018 Option Grants | Tranche One | Henry R. Slack |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount per share (in dollars per share) |  |  |  |  |  |  |  | $ 35.00 |  |  |  |  |  |  |
Period of consecutive trading days |  |  |  |  |  |  |  | 20 days |  |  |  |  |  |  |
Number of shares expected to vest (in shares) | shares |  |  |  |  |  |  |  | 26,250 |  |  |  |  |  |  |
2018 Option Grants | Tranche Two |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount per share (in dollars per share) |  |  | $ 40.00 |  |  |  |  |  |  |  |  |  |  |  |
Percentage of options |  |  | 25.00% |  |  |  |  |  |  |  |  |  |  |  |
Period of consecutive trading days |  |  | 20 years |  |  |  |  |  |  |  |  |  |  |  |
2018 Option Grants | Tranche Two | Henry R. Slack |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount per share (in dollars per share) |  |  |  |  |  |  |  | $ 40.00 |  |  |  |  |  |  |
Period of consecutive trading days |  |  |  |  |  |  |  | 20 days |  |  |  |  |  |  |
Number of shares expected to vest (in shares) | shares |  |  |  |  |  |  |  | 26,250 |  |  |  |  |  |  |
2018 Option Grants | Tranche Three |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount per share (in dollars per share) |  |  | $ 45.00 |  |  |  |  |  |  |  |  |  |  |  |
Percentage of options |  |  | 25.00% |  |  |  |  |  |  |  |  |  |  |  |
Period of consecutive trading days |  |  | 20 years |  |  |  |  |  |  |  |  |  |  |  |
2018 Option Grants | Tranche Four |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount per share (in dollars per share) |  |  | $ 50.00 |  |  |  |  |  |  |  |  |  |  |  |
Percentage of options |  |  | 25.00% |  |  |  |  |  |  |  |  |  |  |  |
Period of consecutive trading days |  |  | 20 years |  |  |  |  |  |  |  |  |  |  |  |
2018 Option Grants | Employee Stock Option |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Grants in period, weighted average (in dollars per share) |  |  |  |  |  |  |  |  |  | $ 7.40 |  |  |  |  |
2018 Option Grants | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share price (in dollars per share) |  |  |  |  |  |  |  |  |  | $ 35.00 |  |  |  |  |
2016 Option Grants |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares issued (in dollars per share) |  |  |  |  |  | $ 27.15 |  |  |  |  |  |  |  |  |
Period following an executive's termination of employment for a number of reasons |  |  |  |  |  | 18 months |  |  |  |  |  |  |  |  |
Period of consecutive trading days |  |  |  |  |  | 20 days |  |  |  |  |  |  |  |  |
2016 Option Grants | Remy W. Trafelet |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options granted in period (in shares) | shares |  |  |  |  |  | 300,000 |  |  |  |  |  |  |  |  |
2016 Option Grants | Henry R. Slack |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized expense | $ |  |  |  |  |  |  |  |  |  | $ 783,000 |  |  |  |  |
Options granted in period (in shares) | shares |  |  |  |  |  | 225,000 |  |  |  |  |  |  |  |  |
Shares forfeited (in shares) | shares |  |  |  |  |  | 187,500 |  |  |  |  |  |  |  |  |
2016 Option Grants | George R. Brokaw |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options granted in period (in shares) | shares |  |  |  |  |  | 225,000 |  |  |  |  |  |  |  |  |
2016 Option Grants | Tranche One |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount per share (in dollars per share) |  |  |  |  |  | $ 60.00 |  |  |  |  |  |  |  |  |
Percentage of options |  |  |  |  |  | 25.00% |  |  |  |  |  |  |  |  |
2016 Option Grants | Tranche Two |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount per share (in dollars per share) |  |  |  |  |  | $ 75.00 |  |  |  |  |  |  |  |  |
Percentage of options |  |  |  |  |  | 25.00% |  |  |  |  |  |  |  |  |
2016 Option Grants | Tranche Three |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount per share (in dollars per share) |  |  |  |  |  | $ 90.00 |  |  |  |  |  |  |  |  |
Percentage of options |  |  |  |  |  | 25.00% |  |  |  |  |  |  |  |  |
2016 Option Grants | Tranche Four |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount per share (in dollars per share) |  |  |  |  |  | $ 105.00 |  |  |  |  |  |  |  |  |
Percentage of options |  |  |  |  |  | 25.00% |  |  |  |  |  |  |  |  |
2016 Option Grants | Employee Stock Option |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Grants in period, weighted average (in dollars per share) |  |  |  |  |  |  |  |  |  |  | $ 3.53 |  |  |  |
2015 Option Grants | Employee Stock Option |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common shares available (in shares) | shares |  |  |  |  |  |  |  | 557,500 |  |  |  |  |  |  |

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