Real Estate and Lending Activities - Concentrations of Credit Risk - Additional Information (Detail) - Investment	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017
Total Gross Assets [Member] | Geographic Concentration [Member] | U.S. [Member]
Business Acquisition [Line Items]
Percentage of revenue from affiliates of total revenue	80.00%
Total Gross Assets [Member] | Geographic Concentration [Member] | Europe [Member]
Business Acquisition [Line Items]
Percentage of revenue from affiliates of total revenue	20.00%
Revenue [Member] | Credit Concentration Risk [Member] | Steward [Member]
Business Acquisition [Line Items]
Percentage of revenue from affiliates of total revenue	37.00%	23.00%
Revenue [Member] | Credit Concentration Risk [Member] | Prime [Member]
Business Acquisition [Line Items]
Percentage of revenue from affiliates of total revenue	16.00%	19.00%
Revenue [Member] | Credit Concentration Risk [Member] | MEDIAN [Member]
Business Acquisition [Line Items]
Percentage of revenue from affiliates of total revenue	16.00%	15.00%
Revenue [Member] | Credit Concentration Risk [Member] | Ernest [Member]
Business Acquisition [Line Items]
Percentage of revenue from affiliates of total revenue	9.00%	11.00%
Revenue [Member] | Customer Concentration Risk | General Acute Care Hospital and Healthcare System [Member]
Business Acquisition [Line Items]
Percentage of revenue from affiliates of total revenue	72.00%
Revenue [Member] | Customer Concentration Risk | Rehabilitation Hospital with Covenant Health System [Member]
Business Acquisition [Line Items]
Percentage of revenue from affiliates of total revenue	24.00%
Revenue [Member] | Customer Concentration Risk | Long-term Acute Care Hospital [Member]
Business Acquisition [Line Items]
Percentage of revenue from affiliates of total revenue	4.00%
Pro Forma [Member] | Total Gross Assets [Member] | Customer Concentration Risk
Business Acquisition [Line Items]
Number of investment in property	0			0
Maximum percentage of entity's assets invested on single property	4.00%			4.00%
Scenario, Forecast [Member] | Total Gross Assets [Member] | Geographic Concentration [Member] | U.S. [Member]
Business Acquisition [Line Items]
Percentage of revenue from affiliates of total revenue			80.00%
Scenario, Forecast [Member] | Total Gross Assets [Member] | Geographic Concentration [Member] | Europe [Member]
Business Acquisition [Line Items]
Percentage of revenue from affiliates of total revenue			20.00%