Employee and Agent Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Plan actuarial gain and loss amortization disclosures
Allowable corridor not used for amortization of actuarial gains and losses of qualified pension plan (as a percent)	10.00%
Pension benefits
Components of Net Periodic Benefit Cost (Income)
Service cost	$ 67.1	$ 65.0	$ 63.2
Interest cost	124.4	134.9	120.3
Expected return on plan assets	(144.2)	(155.0)	(160.6)
Amortization of prior service (benefit) cost	(2.3)	(2.2)	(1.9)
Recognized net actuarial (gain) loss	68.0	77.0	102.4
Net periodic benefit cost (income)	113.0	119.7	123.4
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	43.9	12.7
Prior service (benefit) cost	(23.1)
Amortization of net gain (loss)	(68.0)	(77.0)
Amortization of prior service benefit (cost)	2.3	2.2
Total recognized in pre-tax accumulated other comprehensive (income) loss	(44.9)	(62.1)
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	68.1	57.6
Estimated amounts that will be amortized from accumulated other comprehensive income into net periodic benefit cost during the next fiscal year
Estimated net actuarial (gain) loss that will be amortized from accumulated other comprehensive income into net periodic benefit cost during the next fiscal year	67.7
Estimated prior service cost (benefit) that will be amortized from accumulated other comprehensive income into net periodic benefit cost during the next fiscal year	(3.4)
Other postretirement benefits
Components of Net Periodic Benefit Cost (Income)
Service cost	0.1	2.1	2.0
Interest cost	3.9	6.2	6.6
Expected return on plan assets	(27.5)	(32.6)	(33.9)
Amortization of prior service (benefit) cost	(34.6)	(25.9)	(18.5)
Recognized net actuarial (gain) loss		0.2	(0.8)
Plan amendments			5.8
Net periodic benefit cost (income)	(58.1)	(50.0)	$ (38.8)
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	(44.7)	18.8
Amortization of net gain (loss)		(0.2)
Amortization of prior service benefit (cost)	34.6	25.9
Plan amendments		(51.6)
Total recognized in pre-tax accumulated other comprehensive (income) loss	(10.1)	(7.1)
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	(68.2)	$ (57.1)
Estimated amounts that will be amortized from accumulated other comprehensive income into net periodic benefit cost during the next fiscal year
Estimated net actuarial (gain) loss that will be amortized from accumulated other comprehensive income into net periodic benefit cost during the next fiscal year	(1.5)
Estimated prior service cost (benefit) that will be amortized from accumulated other comprehensive income into net periodic benefit cost during the next fiscal year	$ (13.9)