Loans Receivable - Scheduled of loans receivable by property type (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal Outstanding	$ 1,643.8	$ 1,573.8
Fair Value	$ 1,631.0	$ 1,572.1
% of Fair Value	100.00%	100.00%
Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal Outstanding	$ 825.2	$ 769.3
Fair Value	$ 821.0	$ 768.0
Office | Credit Concentration Risk | Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
% of Fair Value	50.30%	48.80%
Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal Outstanding	$ 199.6	$ 199.6
Fair Value	$ 199.6	$ 199.6
Industrial | Credit Concentration Risk | Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
% of Fair Value	12.20%	12.70%
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal Outstanding	$ 139.5	$ 158.5
Fair Value	$ 139.5	$ 158.5
Retail | Credit Concentration Risk | Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
% of Fair Value	8.60%	10.10%
Storage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal Outstanding	$ 82.0	$ 82.0
Fair Value	$ 77.9	$ 82.0
Storage | Credit Concentration Risk | Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
% of Fair Value	4.80%	5.20%
Apartments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal Outstanding	$ 262.2	$ 229.1
Fair Value	$ 260.0	$ 228.8
Apartments | Credit Concentration Risk | Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
% of Fair Value	15.90%	14.60%
Hotel
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal Outstanding	$ 135.3	$ 135.3
Fair Value	$ 133.0	$ 135.2
Hotel | Credit Concentration Risk | Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
% of Fair Value	8.20%	8.60%