Employee and Agent Benefits - Weighted Average Assumptions (Details) - USD ($) $ in Millions	2 Months Ended	10 Months Ended	12 Months Ended
	Dec. 31, 2016	Oct. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension benefits
Weighted-average assumptions used to determine benefit obligations as disclosed under the Obligations and Funded Status section
Discount rate: Benefit obligation (as a percent)	4.15%		3.60%	4.15%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate: Net periodic benefit cost (as a percent)			4.15%	4.50%	4.00%
Expected long-term return on plan assets: Net periodic benefit cost (as a percent)			6.70%	7.20%	7.20%
Pension benefits, Cash balance benefit
Weighted-average assumptions used to determine benefit obligations as disclosed under the Obligations and Funded Status section
Rate of compensation increase: Benefit obligation (as a percent)	5.02%		4.96%	5.02%
Weighted-average assumptions used to determine net periodic benefit cost
Rate of compensation increase: Net periodic benefit cost (as a percent)			5.02%	5.24%	5.25%
Pension benefits, Traditional benefit
Weighted-average assumptions used to determine benefit obligations as disclosed under the Obligations and Funded Status section
Rate of compensation increase: Benefit obligation (as a percent)	2.70%		2.66%	2.70%
Weighted-average assumptions used to determine net periodic benefit cost
Rate of compensation increase: Net periodic benefit cost (as a percent)			2.70%	2.98%	3.02%
Other postretirement benefits
Weighted-average assumptions used to determine benefit obligations as disclosed under the Obligations and Funded Status section
Discount rate: Benefit obligation (as a percent)	3.75%		3.35%	3.75%
Rate of compensation increase: Benefit obligation (as a percent)	2.44%		2.39%	2.44%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate: Net periodic benefit cost (as a percent)	3.35%	4.15%	3.75%		4.00%
Expected long-term return on plan assets: Net periodic benefit cost (as a percent)			4.40%	5.24%	5.36%
Rate of compensation increase: Net periodic benefit cost (as a percent)			2.44%	4.82%	4.82%
Assumed Health Care Cost Trend Rates
Health care cost trend rate assumed for next year under age 65 (as a percent)	7.00%		7.00%	7.00%
Health care cost trend rate assumed for next year age 65 and over (as a percent)	6.00%		7.00%	6.00%
Rate to which the health care cost trend rate is assumed to decline (the ultimate trend rate) (as a percent)	4.50%		4.50%	4.50%
Year that the health care cost trend rate reaches the ultimate trend rate under age 65 (calendar year)			2024	2023
Year that the health care cost trend rate reaches the ultimate trend rate age 65 and over (calendar year)			2024	2021
Effect of one-percentage-point change in assumed health care cost trend rates
Effect on total of service cost and interest cost components of a one-percentage-point increase in assumed health care cost trend rates			$ 0.1
Effect on total of service cost and interest cost components of a one-percentage-point decrease in assumed health care cost trend rates			(0.1)
Effect on accumulated postretirement benefit obligation of a one-percentage-point increase in assumed health care cost trend rates			(1.5)
Effect on accumulated postretirement benefit obligation of a one-percentage-point decrease in assumed health care cost trend rates			$ 1.4
Medical and life plans
Weighted-average assumptions used to determine net periodic benefit cost
Expected long-term return on plan assets: Net periodic benefit cost, under age 65 (as a percent)			4.40%
Expected long-term return on plan assets: Net periodic benefit cost, age 65 and over (as a percent)			4.50%
Long-term care plans
Weighted-average assumptions used to determine net periodic benefit cost
Expected long-term return on plan assets: Net periodic benefit cost (as a percent)			3.75%