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INVESTMENTS - TEXTUAL (Details)
$ in Millions
1 Months Ended 3 Months Ended 12 Months Ended
Mar. 31, 2018
USD ($)
joint_venture
Mar. 31, 2018
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Schedule of Available-for-sale Securities [Line Items]          
Debt securities exposure in single issuer greater than stated percentage of total investments 1.00% 1.00%      
Available-for-sale securities, amortized cost basis       $ 34,988.0  
Gross unrealized losses of twelve months or more $ 404.0 $ 404.0   224.0  
Carrying value of fixed maturities non-income producing 2.0 2.0      
Investment expenses   19.0 $ 19.0    
Trading securities, at fair value 13,457.0 13,457.0   12,628.0  
Separate account equity investment carrying value $ 48.0 48.0   49.0  
Number of real estate joint ventures sold | joint_venture 2        
Total purchase price $ 143.0        
Pre-tax loss on sale of real estate joint venture 0.2        
Elimination of long-term debt 203.0        
Derecognized US Treasury securities 3,673.0 3,673.0     $ 3,905.0
Proceeds from return swap contract         $ 3,905.0
Fair value of return swap contracts 16.0 16.0      
Treasury Lock          
Schedule of Available-for-sale Securities [Line Items]          
Unrealized gain (loss) related to TIPS   (88.0)   (86.0)  
Commercial Mortgage Loans          
Schedule of Available-for-sale Securities [Line Items]          
Mortgage loans classified as nonaccrual 19.0 19.0   19.0  
Other Than Investment Grade          
Schedule of Available-for-sale Securities [Line Items]          
Available-for-sale securities, amortized cost $ 1,280.0 $ 1,280.0   $ 1,309.0  
Percentage of available for sale securities 3.80% 3.80%   3.80%  
Public corporate          
Schedule of Available-for-sale Securities [Line Items]          
Debt securities exposure in single issuer of total investments $ 208.0 $ 208.0   $ 182.0  
Available-for-sale securities, amortized cost basis       13,645.0  
Gross unrealized losses of twelve months or more 27.0 27.0   16.0  
Fixed Maturities          
Schedule of Available-for-sale Securities [Line Items]          
Available-for-sale securities, amortized cost basis $ 33,438.0 33,438.0   34,831.0  
Fixed Maturities | Other Than Investment Grade          
Schedule of Available-for-sale Securities [Line Items]          
Net unrealized losses   $ 14.0   $ 5.0