Leases - Operating Leases (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($) shopping_center	Jun. 30, 2017 USD ($)
Operating Leased Assets [Line Items]
Number of shopping centers with land leases | shopping_center	6
Ground lease expense	$ 0.8	$ 1.8
Rent expense capitalized	0.3	0.2
Rent expense	$ 0.5	$ 0.5
Minimum
Operating Leased Assets [Line Items]
Operating lease term	1 month
Maximum
Operating Leased Assets [Line Items]
Operating lease term	60 years
Period of lease term	22 years