Real Estate and Other Activities - Net Assets Acquired (Parenthetical) (Detail) $ in Thousands, £ in Millions		6 Months Ended
		Jun. 30, 2021 USD ($) Property	Jun. 30, 2020	Mar. 31, 2021 GBP (£)
Business Acquisition [Line Items]
Advanced to the mortgage loan | $	[1]	$ 1,090,400
Priory Group [Member]
Business Acquisition [Line Items]
Number of real estate assets acquired | Property		35
Mortgage Loans [Member] | Priory Group [Member]
Business Acquisition [Line Items]
Advanced to the mortgage loan | £				£ 800
Intangible Lease Assets
Business Acquisition [Line Items]
Weighted-average useful life of acquired intangible lease assets (in years)		45 years	30 years 9 months 18 days

[1] The 2021 column includes an £800 million mortgage loan advanced to the Priory Group (“Priory”) in the first quarter of 2021 and converted to fee simple ownership of 35 properties in the second quarter of 2021 as described below