Leases - Operating Leases (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) shopping_center	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Operating Leased Assets [Line Items]
Number of shopping centers with land leases | shopping_center	7
Ground lease expense	$ 1.4	$ 1.2	$ 1.7
Rent expense capitalized	0.1	0.6	0.9
Rent expense	$ 1.0	$ 1.0	$ 1.4
Minimum
Operating Leased Assets [Line Items]
Period of lease term	25 years
Maximum
Operating Leased Assets [Line Items]
Period of lease term	71 years