Employee and Agent Benefits - Contributions and Estimated Future Benefit Payments (Details) $ in Millions	Dec. 31, 2017 USD ($)
Pension benefits
Estimated Future Benefit Payments
Estimated future benefit payments, Year 1	$ 127.5
Estimated future benefit payments, Year 2	129.4
Estimated future benefit payments, Year 3	139.5
Estimated future benefit payments, Year 4	145.3
Estimated future benefit payments, Year 5	154.5
Estimated future benefit payments, Years 6-10	882.6
Pension benefits | Maximum
Contributions
Possible contributions to be made during the next fiscal year to the qualified and nonqualified pension plans combined	125.0
Other postretirement benefits
Estimated Future Benefit Payments
Estimated future benefit payments, Year 1	13.1
Estimated future benefit payments, Year 2	13.6
Estimated future benefit payments, Year 3	12.5
Estimated future benefit payments, Year 4	11.6
Estimated future benefit payments, Year 5	10.8
Estimated future benefit payments, Years 6-10	$ 9.9