Related Person Transactions - Revenues from Related Parties (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Related Party Transaction [Line Items]
Revenue from related parties	$ 131,564	$ 140,759	$ 288,251	$ 299,922
Revenue from unrelated parties	0	102	259	831
Revenue	131,564	140,861	288,510	300,753
Total management and advisory services revenues
Related Party Transaction [Line Items]
Revenue from related parties	41,991	43,999	83,065	91,400
Revenue from unrelated parties	0	102	259	823
Revenue	41,991	44,101	83,324	92,223
Total reimbursable costs
Related Party Transaction [Line Items]
Revenue from related parties	89,573	96,760	205,186	208,522
Revenue from unrelated parties	0	0	0	8
Revenue	89,573	96,760	205,186	208,530
Managed Equity REITs
Related Party Transaction [Line Items]
Revenue from related parties	113,395	124,951	254,689	268,856
Managed Equity REITs | Total management and advisory services revenues
Related Party Transaction [Line Items]
Revenue from related parties	32,734	35,707	66,014	75,072
Managed Equity REITs | Total reimbursable costs
Related Party Transaction [Line Items]
Revenue from related parties	80,661	89,244	188,675	193,784
DHC
Related Party Transaction [Line Items]
Revenue from related parties	39,553	41,047	90,759	84,604
DHC | Total management and advisory services revenues
Related Party Transaction [Line Items]
Revenue from related parties	8,652	9,145	17,574	19,100
DHC | Total reimbursable costs
Related Party Transaction [Line Items]
Revenue from related parties	30,901	31,902	73,185	65,504
ILPT
Related Party Transaction [Line Items]
Revenue from related parties	8,649	10,274	18,714	26,615
ILPT | Total management and advisory services revenues
Related Party Transaction [Line Items]
Revenue from related parties	4,173	5,305	9,047	10,879
ILPT | Total reimbursable costs
Related Party Transaction [Line Items]
Revenue from related parties	4,476	4,969	9,667	15,736
OPI
Related Party Transaction [Line Items]
Revenue from related parties	50,558	56,569	115,352	121,452
OPI | Total management and advisory services revenues
Related Party Transaction [Line Items]
Revenue from related parties	8,628	9,480	17,895	19,643
OPI | Total reimbursable costs
Related Party Transaction [Line Items]
Revenue from related parties	41,930	47,089	97,457	101,809
SVC
Related Party Transaction [Line Items]
Revenue from related parties	14,635	17,061	29,864	36,185
SVC | Total management and advisory services revenues
Related Party Transaction [Line Items]
Revenue from related parties	11,281	11,777	21,498	25,450
SVC | Total reimbursable costs
Related Party Transaction [Line Items]
Revenue from related parties	3,354	5,284	8,366	10,735
Managed REITs
Related Party Transaction [Line Items]
Revenue from related parties	116,757	126,352	259,603	271,765
Managed REITs | Total management and advisory services revenues
Related Party Transaction [Line Items]
Revenue from related parties	34,483	36,487	68,349	76,699
Managed REITs | Total reimbursable costs
Related Party Transaction [Line Items]
Revenue from related parties	82,274	89,865	191,254	195,066
RMRM
Related Party Transaction [Line Items]
Revenue from related parties	1,692	743	2,241	1,554
RMRM | Total management and advisory services revenues
Related Party Transaction [Line Items]
Revenue from related parties	751	743	1,300	1,554
RMRM | Total reimbursable costs
Related Party Transaction [Line Items]
Revenue from related parties	941	0	941	0
TRMT
Related Party Transaction [Line Items]
Revenue from related parties	1,670	658	2,673	1,355
TRMT | Total management and advisory services revenues
Related Party Transaction [Line Items]
Revenue from related parties	998	37	1,035	73
TRMT | Total reimbursable costs
Related Party Transaction [Line Items]
Revenue from related parties	672	621	1,638	1,282
Managed Private Real Estate Capital
Related Party Transaction [Line Items]
Revenue from related parties	9,108	7,868	16,930	15,272
Managed Private Real Estate Capital | Total management and advisory services revenues
Related Party Transaction [Line Items]
Revenue from related parties	2,134	1,215	3,704	2,278
Managed Private Real Estate Capital | Total reimbursable costs
Related Party Transaction [Line Items]
Revenue from related parties	6,974	6,653	13,226	12,994
Managed Operators
Related Party Transaction [Line Items]
Revenue from related parties	5,699	6,539	11,718	12,885
Managed Operators | Total management and advisory services revenues
Related Party Transaction [Line Items]
Revenue from related parties	5,374	6,297	11,012	12,423
Managed Operators | Total reimbursable costs
Related Party Transaction [Line Items]
Revenue from related parties	325	242	706	462
Five Star
Related Party Transaction [Line Items]
Revenue from related parties	1,918	2,450	3,964	4,726
Five Star | Total management and advisory services revenues
Related Party Transaction [Line Items]
Revenue from related parties	1,803	2,351	3,779	4,603
Five Star | Total reimbursable costs
Related Party Transaction [Line Items]
Revenue from related parties	115	99	185	123
Sonesta
Related Party Transaction [Line Items]
Revenue from related parties	715	643	1,068	1,268
Sonesta | Total management and advisory services revenues
Related Party Transaction [Line Items]
Revenue from related parties	636	567	989	1,146
Sonesta | Total reimbursable costs
Related Party Transaction [Line Items]
Revenue from related parties	79	76	79	122
TA
Related Party Transaction [Line Items]
Revenue from related parties	3,066	3,446	6,686	6,891
TA | Total management and advisory services revenues
Related Party Transaction [Line Items]
Revenue from related parties	2,935	3,379	6,244	6,674
TA | Total reimbursable costs
Related Party Transaction [Line Items]
Revenue from related parties	$ 131	$ 67	$ 442	$ 217