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Equity-Based Compensation - Narrative (Details) |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |
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Jul. 19, 2018 USD ($) $ / shares shares | Jun. 02, 2015 shares | Sep. 30, 2019 USD ($) shares | Sep. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) shares | Sep. 30, 2018 USD ($) shares | Dec. 31, 2018 shares | Feb. 28, 2019 USD ($) | Jan. 31, 2019 USD ($) | Jan. 01, 2019 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Equity-based compensation |  |  | $ 2,501,000 | $ 2,052,000 | $ 7,039,000 | $ 1,198,000 |  |  |  |
Distributions paid |  |  |  |  | $ (405,000) | $ (448,000) |  |  |  |
Director |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Shares issued in lieu of cash (shares) | shares |  |  |  |  | 0 | 0 |  |  |  |
Incentive Restricted Share Plan |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Annual retainer payable, cash |  |  | 50.00% |  | 50.00% |  |  |  |  |
Annual retainer payable, restricted stock units |  |  | 50.00% |  | 50.00% |  |  |  |  |
Incentive Restricted Share Plan | Independent Directors |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Annual retainer payable |  |  | $ 100,000 |  | $ 100,000 |  |  |  |  |
Incentive Restricted Share Plan | Non-Executive Chair |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Annual retainer payable |  |  | 105,000 |  | 105,000 |  |  |  |  |
Incentive Restricted Share Plan | Directors, Servicing on Committees |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Annual retainer payable |  |  | $ 30,000 |  | $ 30,000 |  |  |  |  |
2015 OPP |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Shares issued during the period (shares) | shares |  | 3,013,933 |  |  |  |  |  |  |  |
Requisite service period |  |  |  |  | 2 years 9 months 18 days |  |  |  |  |
Distribution percent entitled to by LTIP holders |  |  | 10.00% |  | 10.00% |  |  |  |  |
Award value, percent of market capitalization |  | 5.00% |  |  |  |  |  |  |  |
2018 Multi Year Outperformance Plan |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Maximum award value | $ 50,000,000.0 |  |  |  |  |  |  |  |  |
Share price (usd per share) | $ / shares | $ 19.57 |  |  |  |  |  |  |  |  |
2018 Multi Year Outperformance Plan | Minimum | Tier One |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Absolute TSR | 24.00% |  |  |  |  |  |  |  |  |
Relative TSR excess | (0.06) |  |  |  |  |  |  |  |  |
2018 Multi Year Outperformance Plan | Minimum | Tier Two |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Absolute TSR | 36.00% |  |  |  |  |  |  |  |  |
Relative TSR excess | 0.00 |  |  |  |  |  |  |  |  |
2018 Multi Year Outperformance Plan | Maximum | Tier One |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Absolute TSR | 30.00% |  |  |  |  |  |  |  |  |
Relative TSR excess | 0.00 |  |  |  |  |  |  |  |  |
2018 Multi Year Outperformance Plan | Maximum | Tier Two |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Absolute TSR | 30.00% |  |  |  |  |  |  |  |  |
Relative TSR excess | 0.06 |  |  |  |  |  |  |  |  |
Stock Options | Stock Option Plan |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Number of shares authorized (shares) | shares |  |  | 500,000 |  | 500,000 |  |  |  |  |
Shares issued during the period (shares) | shares |  |  |  |  | 0 |  | 0 |  |  |
Restricted Stock Units (RSUs) | Incentive Restricted Share Plan |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Percentage of maximum Common Stock available for issuance |  |  |  |  | 10.00% |  |  |  |  |
Vesting period |  |  |  |  | 3 years |  |  |  |  |
Restricted Stock | Incentive Restricted Share Plan |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Share-based compensation expense |  |  | $ 100,000 | 100,000 | $ 300,000 | $ 400,000 |  |  |  |
Restricted Stock | Restricted Share Plan |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Compensation expense |  |  | 700,000 |  | $ 700,000 |  |  |  |  |
Weighted average period of recognition |  |  |  |  | 2 years 1 month 6 days |  |  |  |  |
LTIP Units |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Distributions paid |  |  |  |  | $ (400,000) | (400,000) |  |  |  |
LTIP Units | 2018 Multi Year Outperformance Plan | Advisor |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Shares issued during the period (shares) | shares | 2,554,930 |  |  |  |  |  |  |  |  |
Share-Based Payment Arrangement, Nonemployee | 2015 OPP |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Requisite service period |  |  |  |  | 2 years 9 months 18 days |  |  |  |  |
Share-Based Payment Arrangement, Nonemployee | LTIP Units | 2015 OPP |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Equity fair value |  |  |  |  |  |  |  |  |  | $ 18,800,000
Share-Based Payment Arrangement, Nonemployee | LTIP Units | 2018 Multi Year Outperformance Plan Amendment |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Share-based compensation expense |  |  |  | 1,900,000 |  | 800,000 |  |  |  |
Equity fair value |  |  |  |  |  |  |  | $ 29,900,000 | $ 23,300,000 |
Fair value in excess |  |  | 6,600,000 |  | $ 6,600,000 |  |  |  |  |
Non-controlling interest |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Equity-based compensation |  |  | 2,359,000 | $ 1,923,000 | 6,710,000 | $ 846,000 |  |  |  |
Non-controlling interest | Share-Based Payment Arrangement, Nonemployee |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Equity-based compensation |  |  | $ 2,400,000 |  | $ 6,700,000 |  |  |  |  |
Below Threshold | 2015 OPP |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Vesting period |  | 1 year |  |  |  |  |  |  |  |
Below Threshold | 2018 Multi Year Outperformance Plan |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Absolute TSR | 24.00% |  |  |  |  |  |  |  |  |
Relative TSR excess | (0.06) |  |  |  |  |  |  |  |  |
Below Threshold | LTIP Units |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Award vesting percent |  |  |  |  | 33.33% |  |  |  |  |
Threshold | 2015 OPP |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Vesting period |  | 2 years |  |  |  |  |  |  |  |
Threshold | 2018 Multi Year Outperformance Plan |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Absolute TSR | 24.00% |  |  |  |  |  |  |  |  |
Relative TSR excess | (0.06) |  |  |  |  |  |  |  |  |
Threshold | LTIP Units |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Award vesting percent |  |  |  |  | 33.33% |  |  |  |  |
Target | 2015 OPP |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Award performance period |  | 3 years |  |  |  |  |  |  |  |
Vesting period |  | 3 years |  |  |  |  |  |  |  |
Target | 2018 Multi Year Outperformance Plan |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Absolute TSR | 30.00% |  |  |  |  |  |  |  |  |
Relative TSR excess | 0 |  |  |  |  |  |  |  |  |
Target | LTIP Units |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Award vesting percent |  |  |  |  | 33.33% |  |  |  |  |
Maximum | 2018 Multi Year Outperformance Plan |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Absolute TSR | 36.00% |  |  |  |  |  |  |  |  |
Relative TSR excess | 0.06 |  |  |  |  |  |  |  |  |

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