Related Party Transactions (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) property	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Forever 21, J.C. Penny and SPARC Group | Lease Income
Related Party Transactions
Amounts charged to related party	$ 54.1	$ 20.9	$ 20.0
Melvin Simon & Associates, Inc. | Amounts for services provided
Related Party Transactions
Amounts charged to related party	$ 0.6	0.6	0.6
Properties owned by related parties | Amounts for services provided
Related Party Transactions
Number of shopping centers owned by related parties in which management services are provided | property	2
Amounts charged to related party	$ 3.3	3.9	4.2
Unconsolidated Joint Ventures | Amounts for services provided
Related Party Transactions
Amounts charged to related party	92.7	108.2	111.5
Unconsolidated Joint Ventures | Development, royalty and other fees
Related Party Transactions
Amounts charged to related party	$ 13.1	$ 14.8	$ 16.0