Mortgage Loans Payable - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($)
Mortgage Loans Payable [Line Items]
Principal balances	$ 2,316.3	$ 2,316.5
Second Street
Mortgage Loans Payable [Line Items]
Mortgage loans on real estate, number of individual loans | loan	3
Principal balances	$ 137.5	$ 137.5
Second Street | Loan One
Mortgage Loans Payable [Line Items]
Principal balances	79.0
Second Street | Loan Two
Mortgage Loans Payable [Line Items]
Principal balances	45.0
Second Street | Loan Three
Mortgage Loans Payable [Line Items]
Principal balances	$ 13.5