Leases - Operating Leases (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($) shopping_center	Sep. 30, 2016 USD ($)
Operating Leased Assets [Line Items]
Number of shopping centers with land leases | shopping_center	7
Ground lease expense	$ 2.8	$ 1.7
Rent expense capitalized	0.4	0.2
Rent expense	$ 0.7	$ 0.7
Minimum
Operating Leased Assets [Line Items]
Period of lease term	25 years
Maximum
Operating Leased Assets [Line Items]
Period of lease term	71 years