Loans Receivable - Scheduled of loans receivable by property type (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair Value	$ 1,105.1	$ 913.0
Percentage	100.00%	100.00%
Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair Value	$ 651.3	$ 512.1
Office | Credit Concentration Risk | Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage	58.80%	56.20%
Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair Value	$ 209.5	$ 176.4
Industrial | Credit Concentration Risk | Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage	19.00%	19.30%
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair Value	$ 101.6	$ 101.6
Retail | Credit Concentration Risk | Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage	9.20%	11.10%
Storage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair Value	$ 82.4	$ 63.2
Storage | Credit Concentration Risk | Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage	7.50%	6.90%
Apartments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair Value	$ 60.3	$ 59.7
Apartments | Credit Concentration Risk | Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage	5.50%	6.50%