Mortgage Loans Payable - Narrative (Details) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($) loan	Dec. 31, 2018 USD ($)
Mortgage Loans Payable [Line Items]
Principal balances by loan	$ 2,788.5	$ 2,688.1
Second Street
Mortgage Loans Payable [Line Items]
Mortgage loans on real estate, number of individual loans | loan	3
Principal balances by loan	$ 137.5	$ 137.5
Second Street | Loan One
Mortgage Loans Payable [Line Items]
Principal balances by loan	79.0
Second Street | Loan Two
Mortgage Loans Payable [Line Items]
Principal balances by loan	45.0
Second Street | Loan Three
Mortgage Loans Payable [Line Items]
Principal balances by loan	$ 13.5