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Related Person Transactions - Revenues from Related Parties (Details) - USD ($) $ in Thousands |  |  |  | 12 Months Ended
--- | --- | --- | --- | ---
May 15, 2023 | Jun. 29, 2022 | Dec. 23, 2021 | Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2022
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues |  |  |  | $ 193,920 | $ 236,164 | $ 199,980
Total reimbursable costs |  |  |  | 700,776 | 726,152 | 632,523
Net Investment Income |  |  |  | 1,313 | 0 | 0
Rental property revenues |  |  |  | 1,604 | 0 | 0
Total revenues |  |  |  | $ 897,613 | $ 962,316 | $ 832,503
Revenue from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 100.00% | 100.00% | 100.00%
Revenue Not from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 100.00% | 100.00% | 100.00%
Revenue Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 100.00% | 100.00% | 100.00%
DHC | Joint Venture With Institutional Investor |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Percentage of ownership sold |  | 10.00% | 35.00% |  |  |
Ownership percentage |  | 10.00% | 20.00% |  |  |
Termination fee |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues | $ 45,282 |  |  |  | $ 45,282 |
Related Party |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues |  |  |  | $ 193,920 | 236,033 | $ 199,881
Total reimbursable costs |  |  |  | 700,776 | 726,152 | 632,523
Total revenues |  |  |  | $ 894,696 | $ 962,185 | $ 832,404
Related Party | Revenue from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 100.00% | 99.90% | 100.00%
Related Party | Revenue Not from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 100.00% | 100.00% | 100.00%
Related Party | Revenue Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 99.70% | 100.00% | 100.00%
Perpetual Capital: |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues |  |  |  | $ 140,648 | $ 199,617 | $ 168,550
Total reimbursable costs |  |  |  | 617,765 | 656,632 | 575,098
Total revenues |  |  |  | $ 758,413 | $ 856,249 | $ 743,648
Perpetual Capital: | Revenue from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 72.60% | 84.50% | 84.30%
Perpetual Capital: | Revenue Not from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 88.20% | 90.40% | 90.90%
Perpetual Capital: | Revenue Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 84.60% | 89.00% | 89.40%
Total Managed Equity REITs |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues |  |  |  | $ 134,882 | $ 139,215 | $ 148,094
Total reimbursable costs |  |  |  | 611,701 | 648,291 | 567,346
Total revenues |  |  |  | $ 746,583 | $ 787,506 | $ 715,440
Total Managed Equity REITs | Revenue from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 69.60% | 58.90% | 74.00%
Total Managed Equity REITs | Revenue Not from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 87.30% | 89.20% | 89.70%
Total Managed Equity REITs | Revenue Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 83.30% | 81.90% | 86.00%
DHC |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues |  |  |  | $ 24,516 | $ 23,675 | $ 30,343
Total reimbursable costs |  |  |  | 127,119 | 156,224 | 157,770
Total revenues |  |  |  | $ 151,635 | $ 179,899 | $ 188,113
DHC | Revenue from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 12.60% | 10.00% | 15.20%
DHC | Revenue Not from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 18.10% | 21.40% | 24.90%
DHC | Revenue Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 16.90% | 18.70% | 22.60%
ILPT |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues |  |  |  | $ 36,704 | $ 36,834 | $ 31,354
Total reimbursable costs |  |  |  | 35,768 | 40,438 | 33,593
Total revenues |  |  |  | $ 72,472 | $ 77,272 | $ 64,947
ILPT | Revenue from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 18.90% | 15.50% | 15.60%
ILPT | Revenue Not from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 5.10% | 5.60% | 5.40%
ILPT | Revenue Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 8.10% | 8.00% | 7.80%
OPI |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues |  |  |  | $ 29,903 | $ 38,163 | $ 42,204
Total reimbursable costs |  |  |  | 212,054 | 334,208 | 308,139
Total revenues |  |  |  | $ 241,957 | $ 372,371 | $ 350,343
OPI | Revenue from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 15.50% | 16.20% | 21.10%
OPI | Revenue Not from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 30.30% | 46.00% | 48.70%
OPI | Revenue Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 27.00% | 38.70% | 42.10%
SVC |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues |  |  |  | $ 43,759 | $ 40,543 | $ 44,193
Total reimbursable costs |  |  |  | 236,760 | 117,421 | 67,844
Total revenues |  |  |  | $ 280,519 | $ 157,964 | $ 112,037
SVC | Revenue from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 22.60% | 17.20% | 22.10%
SVC | Revenue Not from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 33.80% | 16.20% | 10.70%
SVC | Revenue Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 31.30% | 16.50% | 13.50%
SEVN |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues |  |  |  | $ 5,766 | $ 5,188 | $ 4,530
Total reimbursable costs |  |  |  | 6,064 | 4,865 | 5,692
Total revenues |  |  |  | $ 11,830 | $ 10,053 | $ 10,222
SEVN | Revenue from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 3.00% | 2.20% | 2.30%
SEVN | Revenue Not from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 0.90% | 0.70% | 0.90%
SEVN | Revenue Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 1.30% | 1.00% | 1.20%
TA |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues |  |  |  |  | $ 55,214 | $ 15,926
Total reimbursable costs |  |  |  |  | 3,476 | 2,060
Total revenues |  |  |  |  | $ 58,690 | $ 17,986
TA | Revenue from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  |  | 23.40% | 8.00%
TA | Revenue Not from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  |  | 0.50% | 0.30%
TA | Revenue Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  |  | 6.10% | 2.20%
Private Capital: |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues |  |  |  | $ 53,272 | $ 36,416 | $ 31,331
Total reimbursable costs |  |  |  | 83,011 | 69,520 | 57,425
Total revenues |  |  |  | $ 136,283 | $ 105,936 | $ 88,756
Private Capital: | Revenue from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 27.40% | 15.40% | 15.70%
Private Capital: | Revenue Not from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 11.80% | 9.60% | 9.10%
Private Capital: | Revenue Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 15.10% | 11.00% | 10.60%
AlerisLife |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues |  |  |  | $ 5,632 | $ 5,414 | $ 4,908
Total reimbursable costs |  |  |  | 0 | 97 | 309
Total revenues |  |  |  | $ 5,632 | $ 5,511 | $ 5,217
AlerisLife | Revenue from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 2.90% | 2.30% | 2.50%
AlerisLife | Revenue Not from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 0.00% | 0.00% | 0.00%
AlerisLife | Revenue Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 0.60% | 0.60% | 0.60%
Sonesta |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues |  |  |  | $ 9,362 | $ 9,471 | $ 8,726
Total reimbursable costs |  |  |  | 0 | 544 | 396
Total revenues |  |  |  | $ 9,362 | $ 10,015 | $ 9,122
Sonesta | Revenue from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 4.80% | 4.00% | 4.40%
Sonesta | Revenue Not from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 0.00% | 0.10% | 0.10%
Sonesta | Revenue Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 1.00% | 1.00% | 1.10%
RMR Residential |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues |  |  |  | $ 16,936 |  |
Total reimbursable costs |  |  |  | 23,369 |  |
Total revenues |  |  |  | $ 40,305 |  |
RMR Residential | Revenue from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 8.70% |  |
RMR Residential | Revenue Not from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 3.30% |  |
RMR Residential | Revenue Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 4.50% |  |
Other private entities |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues |  |  |  | $ 21,342 | $ 21,531 | $ 17,697
Total reimbursable costs |  |  |  | 59,642 | 68,879 | 56,720
Total revenues |  |  |  | $ 80,984 | $ 90,410 | $ 74,417
Other private entities | Revenue from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 11.00% | 9.10% | 8.80%
Other private entities | Revenue Not from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 8.50% | 9.50% | 9.00%
Other private entities | Revenue Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 9.00% | 9.40% | 8.90%
Revenues from unrelated parties |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Total revenues |  |  |  | $ 0 | $ 131 | $ 99
Total reimbursable costs |  |  |  | 0 | 0 | 0
Total revenues |  |  |  | $ 2,917 | $ 131 | $ 99
Revenues from unrelated parties | Revenue from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 0.00% | 0.10% | 0.00%
Revenues from unrelated parties | Revenue Not from Contract with Customer Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 0.00% | 0.00% | 0.00%
Revenues from unrelated parties | Revenue Benchmark | Related Party Concentration Risk |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  |  | 0.00% | 0.00%
Revenues from unrelated parties | Revenue Benchmark | Non-Related Party Concentration |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 0.30% |  |
Revenues from unrelated parties | Revenue Benchmark | Non-Related Party, Investment Income Concentration |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 0.10% |  |
Revenues from unrelated parties | Revenue Benchmark | Non-Related Party, Revenue Concentration |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Concentration percentage |  |  |  | 0.20% |  |

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