Leases - Operating Leases (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) shopping_center	Mar. 31, 2017 USD ($)
Operating Leased Assets [Line Items]
Number of shopping centers with land leases | shopping_center	7
Ground lease expense	$ 0.4	$ 0.8
Rent expense capitalized	0.2	0.1
Rent expense	$ 0.2	$ 0.2
Minimum
Operating Leased Assets [Line Items]
Operating lease term	1 month
Period of lease term	25 years
Maximum
Operating Leased Assets [Line Items]
Operating lease term	99 years
Period of lease term	71 years