Employee and Agent Benefits - Target Plan Asset Allocation (Details)	Dec. 31, 2017
Pension benefits | Qualified benefit plans | Fixed income security portfolios | Minimum
Asset category target allocation
Asset category target allocation	25.00%
Pension benefits | Qualified benefit plans | Fixed income security portfolios | Maximum
Asset category target allocation
Asset category target allocation	80.00%
Pension benefits | Qualified benefit plans | Equity portfolios | Minimum
Asset category target allocation
Asset category target allocation	5.00%
Pension benefits | Qualified benefit plans | Equity portfolios | Maximum
Asset category target allocation
Asset category target allocation	60.00%
Pension benefits | Qualified benefit plans | Real estate investment portfolios
Asset category target allocation
Asset category target allocation	10.00%
Pension benefits | Qualified benefit plans | Alternatives
Asset category target allocation
Asset category target allocation	5.00%
Other postretirement benefits | U.S. equity portfolios
Asset category target allocation
Asset category target allocation	24.00%
Other postretirement benefits | International equity portfolios
Asset category target allocation
Asset category target allocation	15.00%
Other postretirement benefits | Fixed income security portfolios
Asset category target allocation
Asset category target allocation	32.00%
Other postretirement benefits | Alternative mutual fund portfolios
Asset category target allocation
Asset category target allocation	24.00%
Other postretirement benefits | Real estate investment portfolios
Asset category target allocation
Asset category target allocation	5.00%