Rent Expense - (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) properties healthcareproperty	Dec. 31, 2016 USD ($)
Operating Leased Assets [Line Items]
Number of properties pursuant to parking lease		2
Number Of properties subject to air space lease | healthcareproperty		1
Number of properties pursuant to ground leases		78
Number of office space leases		4
Maximum lease terms		98 years
Parking, Air And Ground Leases | Operating Expense
Operating Leased Assets [Line Items]
Rent expenses for parking and ground leases | $	$ 1.3	$ 2.4	$ 1.9
Office Leases | Operating Expense
Operating Leased Assets [Line Items]
Rent expenses for parking and ground leases | $		$ 0.1