v3.8.0.1
Employee Benefit Plans (Changes in Fair Value of Level 3 Pension and Post Retirement Assets) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Pooled Separate Accounts | Fixed Maturities | Pension Benefits    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair Value, beginning of period $ 36 $ 35
Actual Return on Assets:    
Relating to assets still held at the reporting date 2 1
Relating to assets sold during the period 0 0
Purchases, sales and settlements 0 0
Transfers in and/or out of Level 3 0 0
Fair Value, end of period 38 36
Pooled Separate Accounts | Real Estate | Pension Benefits    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair Value, beginning of period 666 607
Actual Return on Assets:    
Relating to assets still held at the reporting date 50 61
Relating to assets sold during the period 6 6
Purchases, sales and settlements (8) (8)
Transfers in and/or out of Level 3 0 0
Fair Value, end of period 714 666
Corporate Debt | Fixed Maturities | Pension Benefits    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair Value, beginning of period 0  
Actual Return on Assets:    
Relating to assets still held at the reporting date 0  
Relating to assets sold during the period 0  
Purchases, sales and settlements 0  
Transfers in and/or out of Level 3 1  
Fair Value, end of period 1 0
Corporate Debt | Asset-backed | Fixed Maturities | Other Postretirement Benefits    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair Value, beginning of period 1 0
Actual Return on Assets:    
Relating to assets still held at the reporting date 0 0
Relating to assets sold during the period 0 0
Purchases, sales and settlements 0 1
Transfers in and/or out of Level 3 (1) 0
Fair Value, end of period 0 1
Collateralized Mortgage Obligations | Corporate Debt | Fixed Maturities | Other Postretirement Benefits    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair Value, beginning of period 5 0
Actual Return on Assets:    
Relating to assets still held at the reporting date 0 0
Relating to assets sold during the period 0 0
Purchases, sales and settlements (3) 5
Transfers in and/or out of Level 3 0 0
Fair Value, end of period 2 5
Collateralized loan obligations | Corporate Debt | Fixed Maturities | Other Postretirement Benefits    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair Value, beginning of period 0  
Actual Return on Assets:    
Relating to assets still held at the reporting date 0  
Relating to assets sold during the period 0  
Purchases, sales and settlements 2  
Transfers in and/or out of Level 3 0  
Fair Value, end of period 2 0
Other | Fixed Maturities | Pension Benefits    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair Value, beginning of period 49 93
Actual Return on Assets:    
Relating to assets still held at the reporting date 0 0
Relating to assets sold during the period 0 0
Purchases, sales and settlements (10) (44)
Transfers in and/or out of Level 3 0 0
Fair Value, end of period 39 49
Other | Fixed Maturities | Other Postretirement Benefits    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair Value, beginning of period 5 3
Actual Return on Assets:    
Relating to assets still held at the reporting date 0 0
Relating to assets sold during the period 0 0
Purchases, sales and settlements 0 2
Transfers in and/or out of Level 3 0 0
Fair Value, end of period $ 5 $ 5