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Fair Value Measurements - Fair Value Option on Consolidated VIEs and Equity Method Investments (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
item
Dec. 31, 2016
USD ($)
item
Dec. 31, 2015
USD ($)
Commercial Mortgage Loans of Consolidated VIEs      
Fair Value Option, Quantitative Disclosures      
Fair value of assets for which fair value option was elected $ 9.3 $ 12.4  
Contractual principal amounts of assets for which the fair value option was elected $ 9.2 $ 12.0  
Number of loans which are more than 90 days past due or in nonaccrual status | item 0 0  
Pre-tax gain (loss) due to change in fair value of assets and liabilities for which the fair value option was elected $ (0.4) $ (0.1) $ (2.0)
Credit risk portion of pre-tax gain (loss) due to change in fair value of assets for which the fair value option was elected 0.0 0.0 0.0
Interest income 0.9 1.2 3.6
Obligations of Consolidated VIEs      
Fair Value Option, Quantitative Disclosures      
Fair value of liabilities for which the fair value option was elected   59.9  
Aggregate unpaid principal amounts of obligations for which the fair value option was elected   60.0  
Credit risk portion of pre-tax gain (loss) due to change in fair value of liabilities for which the fair value option was elected (0.1) (9.8) (1.9)
Pre-tax gain (loss) due to change in fair value of assets and liabilities for which the fair value option was elected (0.1) (9.8) (2.1)
Interest expense 0.3 1.1 1.1
Real Estate Ventures      
Fair Value Option, Quantitative Disclosures      
Fair value of assets for which fair value option was elected 6.5 36.9  
Pre-tax gain (loss) due to change in fair value of assets and liabilities for which the fair value option was elected 3.8 1.5 $ 7.2
Investment Funds      
Fair Value Option, Quantitative Disclosures      
Fair value of assets for which fair value option was elected 45.2 36.9  
Pre-tax gain (loss) due to change in fair value of assets and liabilities for which the fair value option was elected 1.7 2.8  
Dividend income $ 1.9 $ 0.3