Related Person Transactions - Revenues from Related Parties (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Related Party Transaction [Line Items]
Revenue from related parties	$ 138,615	$ 143,506	$ 438,537	$ 553,718
Revenue from unrelated parties	34	209	865	406
Total revenues	$ 138,649	$ 143,715	$ 439,402	$ 554,124
Percentage of revenues from related parties	100.00%	99.90%	99.80%	99.90%
Percentage of revenues from unrelated parties	0.00%	0.10%	0.20%	0.10%
Percentage of revenue from related parties, net	100.00%	100.00%	100.00%	100.00%
Managed REITs
Related Party Transaction [Line Items]
Revenue from related parties	$ 124,105	$ 125,408	$ 392,961	$ 503,616
Percentage of revenues from related parties	89.50%	87.30%	89.40%	90.90%
DHC
Related Party Transaction [Line Items]
Revenue from related parties	$ 43,472	$ 43,483	$ 128,076	$ 166,313
Percentage of revenues from related parties	31.40%	30.30%	29.10%	30.00%
ILPT
Related Party Transaction [Line Items]
Revenue from related parties	$ 10,034	$ 12,664	$ 36,649	$ 27,998
Percentage of revenues from related parties	7.20%	8.80%	8.30%	5.10%
OPI
Related Party Transaction [Line Items]
Revenue from related parties	$ 58,443	$ 57,374	$ 179,895	$ 171,731
Percentage of revenues from related parties	42.10%	39.90%	41.00%	31.00%
SIR
Related Party Transaction [Line Items]
Revenue from related parties	$ 0	$ 0	$ 0	$ 47,843
Percentage of revenues from related parties	0.00%	0.00%	0.00%	8.60%
SVC
Related Party Transaction [Line Items]
Revenue from related parties	$ 12,156	$ 11,887	$ 48,341	$ 89,731
Percentage of revenues from related parties	8.80%	8.30%	11.00%	16.20%
Managed Operators
Related Party Transaction [Line Items]
Revenue from related parties	$ 5,566	$ 6,802	$ 18,451	$ 20,274
Percentage of revenues from related parties	4.10%	4.70%	4.20%	3.60%
Five Star
Related Party Transaction [Line Items]
Revenue from related parties	$ 2,227	$ 2,466	$ 6,953	$ 7,318
Percentage of revenues from related parties	1.60%	1.70%	1.60%	1.30%
Sonesta
Related Party Transaction [Line Items]
Revenue from related parties	$ 209	$ 881	$ 1,477	$ 2,420
Percentage of revenues from related parties	0.20%	0.60%	0.30%	0.40%
TA
Related Party Transaction [Line Items]
Revenue from related parties	$ 3,130	$ 3,455	$ 10,021	$ 10,536
Percentage of revenues from related parties	2.30%	2.40%	2.30%	1.90%
Other Client Companies
Related Party Transaction [Line Items]
Revenue from related parties	$ 8,944	$ 11,296	$ 27,125	$ 29,828
Percentage of revenues from related parties	6.40%	7.90%	6.20%	5.40%
ABP Trust
Related Party Transaction [Line Items]
Revenue from related parties	$ 3,024	$ 3,476	$ 9,195	$ 10,746
Percentage of revenues from related parties	2.20%	2.50%	2.20%	1.90%
AIC
Related Party Transaction [Line Items]
Revenue from related parties	$ 2	$ 187	$ 98	$ 307
Percentage of revenues from related parties	0.00%	0.10%	0.00%	0.10%
Open End Fund
Related Party Transaction [Line Items]
Revenue from related parties	$ 4,738	$ 5,583	$ 13,653	$ 13,693
Percentage of revenues from related parties	3.40%	3.90%	3.10%	2.50%
RMRM
Related Party Transaction [Line Items]
Revenue from related parties	$ 585	$ 767	$ 2,139	$ 2,225
Percentage of revenues from related parties	0.40%	0.50%	0.50%	0.40%
TRMT
Related Party Transaction [Line Items]
Revenue from related parties	$ 577	$ 1,283	$ 1,932	$ 2,857
Percentage of revenues from related parties	0.40%	0.90%	0.40%	0.50%
Centre Street
Related Party Transaction [Line Items]
Revenue from related parties	$ 18	$ 0	$ 108	$ 0
Percentage of revenues from related parties	0.00%	0.00%	0.00%	0.00%