Fair Value Measurements (Other Fair Value Disclosures) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Level 2 [Member] \| Senior Unsecured Notes [Member]
Other Fair Value Measurements
Senior unsecured notes	$ 974.8	$ 971.6

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
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Name:	us-gaap_DebtInstrumentFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant