Employee and Agent Benefits - Plan Information (Details)	12 Months Ended
	Dec. 31, 2017
Pension benefits
Defined Benefit Plan Disclosures
Eligibility age	21 years
Period of employment used to calculate average annual compensation for the final average pay benefit	5 years
Maximum age used to calculate a partial benefit accrual of final average pay benefits	65 years
Minimum years of potential service used to calculate a partial benefit accrual of final average pay benefits	35 years
Other postretirement benefits
Defined Benefit Plan Disclosures
Eligibility age	57 years
Eligibility service period	10 years
Retiree health benefits
Defined Benefit Plan Disclosures
Participant contributions as a percent of expected cost of coverage for employees hired prior to January 1, 2002, and who retired on or after January 1, 2011 (as a percent)	60.00%