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Summary of Significant Accounting Policies - Equity Instruments and Investments (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Equity Instruments and Investments    
Equity instruments with readily determinable fair value $ 93.7 $ 88.3
Equity instruments without readily determinable fair values 186.7 186.9
Debt securities of our captive insurance companies $ 62.9 55.7
Debt Securities | Securities in captive insurance subsidiary portfolio | Minimum    
Equity Instruments and Investments    
Investment maturity period 1 year  
Debt Securities | Securities in captive insurance subsidiary portfolio | Maximum    
Equity Instruments and Investments    
Investment maturity period 10 years  
Simon Property Group, L.P.    
Equity Instruments and Investments    
Net unrealized losses recorded in other comprehensive income (loss) $ 0.7 $ 0.4