v3.8.0.1
Employee Benefit Plans (Summary of Pension and Postretirement Plan Asset Allocation) (Details)
Dec. 31, 2017
Dec. 31, 2016
Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of asset class to total plan assets (actual) 100.00% 100.00%
Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of asset class to total plan assets (actual) 100.00% 100.00%
Equity securities | U.S. | Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of asset class to total plan assets (actual) 5.00% 4.00%
Equity securities | U.S. | Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of asset class to total plan assets (actual) 49.00% 44.00%
Equity securities | International | Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of asset class to total plan assets (actual) 5.00% 4.00%
Equity securities | International | Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of asset class to total plan assets (actual) 13.00% 12.00%
Fixed Maturities | U.S. | Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of asset class to total plan assets (actual) 66.00% 68.00%
Fixed Maturities | U.S. | Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of asset class to total plan assets (actual) 34.00% 36.00%
Short-term investments | U.S. | Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of asset class to total plan assets (actual) 0.00% 0.00%
Short-term investments | U.S. | Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of asset class to total plan assets (actual) 4.00% 8.00%
Real Estate | U.S. | Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of asset class to total plan assets (actual) 8.00% 8.00%
Real Estate | U.S. | Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of asset class to total plan assets (actual) 0.00% 0.00%
Other | U.S. | Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of asset class to total plan assets (actual) 16.00% 16.00%
Other | U.S. | Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of asset class to total plan assets (actual) 0.00% 0.00%