Related Person Transactions - Revenues from Related Parties (Details) - USD ($) $ in Thousands			3 Months Ended		6 Months Ended
	Jan. 31, 2022	Dec. 23, 2021	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Related Party Transaction [Line Items]
Total revenues from related parties			$ 197,654	$ 131,564	$ 379,169	$ 288,251
Revenues from unrelated parties			0	0	53	259
Total revenues			197,654	131,564	379,222	288,510
DHC | Joint Venture With Institutional Investor
Related Party Transaction [Line Items]
Percentage of ownership sold	80.00%	35.00%
Ownership percentage	20.00%	20.00%
Total management and advisory services revenues
Related Party Transaction [Line Items]
Total revenues from related parties			49,288	41,991	95,250	83,065
Revenues from unrelated parties			0	0	53	259
Total revenues			49,288	41,991	95,303	83,324
Total reimbursable costs
Related Party Transaction [Line Items]
Total revenues from related parties			148,366	89,573	283,919	205,186
Revenues from unrelated parties			0	0	0	0
Total revenues			148,366	89,573	283,919	205,186
Managed Public Real Estate Capital
Related Party Transaction [Line Items]
Total revenues from related parties			170,569	116,757	333,136	259,603
Managed Public Real Estate Capital | Total management and advisory services revenues
Related Party Transaction [Line Items]
Total revenues from related parties			38,008	34,483	75,000	68,349
Managed Public Real Estate Capital | Total reimbursable costs
Related Party Transaction [Line Items]
Total revenues from related parties			132,561	82,274	258,136	191,254
Managed Equity REITs
Related Party Transaction [Line Items]
Total revenues from related parties			168,145	113,395	327,324	254,689
Managed Equity REITs | Total management and advisory services revenues
Related Party Transaction [Line Items]
Total revenues from related parties			36,871	32,734	72,745	66,014
Managed Equity REITs | Total reimbursable costs
Related Party Transaction [Line Items]
Total revenues from related parties			131,274	80,661	254,579	188,675
DHC
Related Party Transaction [Line Items]
Total revenues from related parties			40,124	39,553	84,452	90,759
DHC | Total management and advisory services revenues
Related Party Transaction [Line Items]
Total revenues from related parties			7,404	8,652	16,529	17,574
DHC | Total reimbursable costs
Related Party Transaction [Line Items]
Total revenues from related parties			32,720	30,901	67,923	73,185
ILPT
Related Party Transaction [Line Items]
Total revenues from related parties			12,808	8,649	24,003	18,714
ILPT | Total management and advisory services revenues
Related Party Transaction [Line Items]
Total revenues from related parties			7,227	4,173	11,742	9,047
ILPT | Total reimbursable costs
Related Party Transaction [Line Items]
Total revenues from related parties			5,581	4,476	12,261	9,667
OPI
Related Party Transaction [Line Items]
Total revenues from related parties			80,997	50,558	162,248	115,352
OPI | Total management and advisory services revenues
Related Party Transaction [Line Items]
Total revenues from related parties			10,518	8,628	21,082	17,895
OPI | Total reimbursable costs
Related Party Transaction [Line Items]
Total revenues from related parties			70,479	41,930	141,166	97,457
SVC
Related Party Transaction [Line Items]
Total revenues from related parties			34,216	14,635	56,621	29,864
SVC | Total management and advisory services revenues
Related Party Transaction [Line Items]
Total revenues from related parties			11,722	11,281	23,392	21,498
SVC | Total reimbursable costs
Related Party Transaction [Line Items]
Total revenues from related parties			22,494	3,354	33,229	8,366
SEVN
Related Party Transaction [Line Items]
Total revenues from related parties			2,424	1,692	5,812	2,241
SEVN | Total management and advisory services revenues
Related Party Transaction [Line Items]
Total revenues from related parties			1,137	751	2,255	1,300
SEVN | Total reimbursable costs
Related Party Transaction [Line Items]
Total revenues from related parties			1,287	941	3,557	941
TRMT
Related Party Transaction [Line Items]
Total revenues from related parties			0	1,670	0	2,673
TRMT | Total management and advisory services revenues
Related Party Transaction [Line Items]
Total revenues from related parties			0	998	0	1,035
TRMT | Total reimbursable costs
Related Party Transaction [Line Items]
Total revenues from related parties			0	672	0	1,638
Managed Private Real Estate Capital
Related Party Transaction [Line Items]
Total revenues from related parties			20,365	9,108	32,111	16,930
Managed Private Real Estate Capital | Total management and advisory services revenues
Related Party Transaction [Line Items]
Total revenues from related parties			4,820	2,134	7,220	3,704
Managed Private Real Estate Capital | Total reimbursable costs
Related Party Transaction [Line Items]
Total revenues from related parties			15,545	6,974	24,891	13,226
ABP Trust
Related Party Transaction [Line Items]
Total revenues from related parties			7,520	6,390	14,932	12,663
ABP Trust | Total management and advisory services revenues
Related Party Transaction [Line Items]
Total revenues from related parties			995	1,042	2,091	2,098
ABP Trust | Total reimbursable costs
Related Party Transaction [Line Items]
Total revenues from related parties			6,525	5,348	12,841	10,565
Other private entities
Related Party Transaction [Line Items]
Total revenues from related parties			12,845	2,718	17,179	4,267
Other private entities | Total management and advisory services revenues
Related Party Transaction [Line Items]
Total revenues from related parties			3,825	1,092	5,129	1,606
Other private entities | Total reimbursable costs
Related Party Transaction [Line Items]
Total revenues from related parties			9,020	1,626	12,050	2,661
Managed Operators
Related Party Transaction [Line Items]
Total revenues from related parties			6,720	5,699	13,922	11,718
Managed Operators | Total management and advisory services revenues
Related Party Transaction [Line Items]
Total revenues from related parties			6,460	5,374	13,030	11,012
Managed Operators | Total reimbursable costs
Related Party Transaction [Line Items]
Total revenues from related parties			260	325	892	706
ALR
Related Party Transaction [Line Items]
Total revenues from related parties			1,287	1,918	2,517	3,964
ALR | Total management and advisory services revenues
Related Party Transaction [Line Items]
Total revenues from related parties			1,226	1,803	2,371	3,779
ALR | Total reimbursable costs
Related Party Transaction [Line Items]
Total revenues from related parties			61	115	146	185
Sonesta
Related Party Transaction [Line Items]
Total revenues from related parties			1,825	715	3,639	1,068
Sonesta | Total management and advisory services revenues
Related Party Transaction [Line Items]
Total revenues from related parties			1,787	636	3,601	989
Sonesta | Total reimbursable costs
Related Party Transaction [Line Items]
Total revenues from related parties			38	79	38	79
TA
Related Party Transaction [Line Items]
Total revenues from related parties			3,608	3,066	7,766	6,686
TA | Total management and advisory services revenues
Related Party Transaction [Line Items]
Total revenues from related parties			3,447	2,935	7,058	6,244
TA | Total reimbursable costs
Related Party Transaction [Line Items]
Total revenues from related parties			$ 161	$ 131	$ 708	$ 442