Employee and Agent Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension benefits
Components of Net Periodic Benefit Cost (Income)
Service cost	$ 16.7	$ 16.2	$ 50.3	$ 48.7
Interest cost	31.1	33.7	93.3	101.1
Expected return on plan assets	(36.0)	(38.7)	(108.1)	(116.2)
Amortization of prior service (benefit) cost	(0.6)	(0.6)	(1.7)	(1.7)
Recognized net actuarial (gain) loss	17.0	19.3	51.0	57.8
Net periodic benefit cost (income)	28.2	29.9	84.8	89.7
Contributions
Minimum annual contribution required in current fiscal year for qualified pension plan			0.0
Contributions made by employer to fund qualified and nonqualified pension plans	13.2		83.5
Pension benefits | Maximum
Contributions
Amount of possible contributions to be made during the current fiscal year to the qualified and nonqualified pension plans combined	125.0		125.0
Other postretirement benefits
Components of Net Periodic Benefit Cost (Income)
Service cost		0.6	0.1	1.8
Interest cost	1.0	1.7	3.0	5.0
Expected return on plan assets	(7.0)	(8.1)	(20.7)	(24.4)
Amortization of prior service (benefit) cost	(8.6)	(5.1)	(25.9)	(15.2)
Recognized net actuarial (gain) loss	(0.1)		(0.1)	0.1
Net periodic benefit cost (income)	$ (14.7)	$ (10.9)	$ (43.6)	$ (32.7)