Employee and Agent Benefits - Benefit Obligation, Funded Status, and Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension benefits
Components of Net Periodic Benefit Cost (Income)
Service cost		$ 63.2	$ 54.0	$ 57.1
Interest cost		120.3	117.1	103.8
Expected return on plan assets		(160.6)	(131.9)	(127.4)
Amortization of prior service (benefit) cost		(1.9)	(4.7)	(8.7)
Recognized net actuarial (gain) loss		102.4	50.5	118.5
Net periodic benefit cost (income)		$ 123.4	85.0	143.3
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
Change in benefit obligation
Benefit obligation at beginning of year		$ (3,060.0)	(2,440.2)
Service cost		63.2	54.0	57.1
Interest cost		120.3	117.1	103.8
Actuarial gain (loss)		96.3	(532.9)
Benefits paid		95.0	84.2
Benefit obligation at end of year	$ (3,052.2)	(3,052.2)	(3,060.0)	(2,440.2)
Change in plan assets
Fair value of plan assets at beginning of year		2,218.3	1,925.6
Actual return on plan assets		(50.0)	246.0
Employer contribution		93.0	130.9
Benefits paid		95.0	84.2
Fair value of plan assets at end of year	2,166.3	2,166.3	2,218.3	1,925.6
Amount recognized in statement of financial position
Amount recognized in other liabilities	(885.9)	(885.9)	(841.7)
Total assets (liabilities) recognized in statement of financial position	(885.9)	(885.9)	(841.7)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	771.2	771.2	759.2
Prior service (benefit) cost	(5.1)	(5.1)	(6.9)
Pre-tax accumulated other comprehensive (income) loss	766.1	766.1	752.3
Additional defined benefit plan disclosures
Accumulated benefit obligation	2,799.2	2,799.2	2,854.0
Information for Defined Benefit Plans With an Accumulated Benefit Obligation in Excess of Plan Assets
Projected benefit obligation for defined benefit plans with an accumulated benefit obligation in excess of plan assets	3,052.2	3,052.2	3,060.0
Accumulated benefit obligation for defined benefit plans with an accumulated benefit obligation in excess of plan assets	2,799.2	2,799.2	2,854.0
Fair value of plan assets for defined benefit plans with an accumulated benefit obligation in excess of plan assets	2,166.3	2,166.3	2,218.3
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss		114.3	418.8
Amortization of net gain (loss)		(102.4)	(50.5)
Amortization of prior service benefit (cost)		1.9	4.7
Total recognized in pre-tax accumulated other comprehensive (income) loss		13.8	373.0
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss		137.2	458.0
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		77.0
Estimated prior service cost (benefit) that will be amortized from accumulated other comprehensive income into net periodic benefit cost during the next fiscal year		(2.2)
Qualified defined benefit pension plans
Components of Net Periodic Benefit Cost (Income)
Service cost		54.8	46.8
Interest cost		102.3	99.1
Expected return on plan assets		(160.6)	(131.9)
Amortization of prior service (benefit) cost		(0.8)	(3.1)
Recognized net actuarial (gain) loss		86.3	42.9
Net periodic benefit cost (income)		82.0	53.8
Change in benefit obligation
Service cost		54.8	46.8
Interest cost		102.3	99.1
Change in plan assets
Fair value of plan assets at beginning of year		2,218.3
Fair value of plan assets at end of year	2,166.3	2,166.3	2,218.3
Amount recognized in statement of financial position
Amount recognized in other liabilities	(453.0)	(453.0)	(385.3)
Total assets (liabilities) recognized in statement of financial position	(453.0)	(453.0)	(385.3)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	659.0	659.0	603.1
Prior service (benefit) cost	(2.6)	(2.6)	(3.4)
Pre-tax accumulated other comprehensive (income) loss	656.4	$ 656.4	599.7
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%
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss		$ 142.1	347.8
Amortization of net gain (loss)		(86.3)	(42.9)
Amortization of prior service benefit (cost)		0.8	3.1
Total recognized in pre-tax accumulated other comprehensive (income) loss		56.6	308.0
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss		138.6	361.8
Nonqualified defined benefit pension plans
Components of Net Periodic Benefit Cost (Income)
Service cost		8.4	7.2
Interest cost		18.0	18.0
Amortization of prior service (benefit) cost		(1.1)	(1.6)
Recognized net actuarial (gain) loss		16.1	7.6
Net periodic benefit cost (income)		41.4	31.2
Change in benefit obligation
Service cost		8.4	7.2
Interest cost		18.0	18.0
Amount recognized in statement of financial position
Amount recognized in other liabilities	(432.9)	(432.9)	(456.4)
Total assets (liabilities) recognized in statement of financial position	(432.9)	(432.9)	(456.4)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	112.2	112.2	156.1
Prior service (benefit) cost	(2.5)	(2.5)	(3.5)
Pre-tax accumulated other comprehensive (income) loss	109.7	109.7	152.6
Additional defined benefit plan disclosures
Market value of assets held in Rabbi trusts for benefit of nonqualified pension plan participants	330.8	330.8	329.6
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss		(27.8)	71.0
Amortization of net gain (loss)		(16.1)	(7.6)
Amortization of prior service benefit (cost)		1.1	1.6
Total recognized in pre-tax accumulated other comprehensive (income) loss		(42.8)	65.0
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss		(1.4)	96.2
Other postretirement benefits
Components of Net Periodic Benefit Cost (Income)
Service cost		2.0	1.4	1.0
Interest cost		6.6	6.6	5.7
Expected return on plan assets		(33.9)	(32.6)	(28.8)
Amortization of prior service (benefit) cost		(18.5)	(20.3)	(25.9)
Recognized net actuarial (gain) loss		(0.8)	(3.4)	1.0
Other		5.8
Net periodic benefit cost (income)		$ (38.8)	(48.3)	(47.0)
Eligibility age		57 years
Eligibility service period		10 years
Change in benefit obligation
Benefit obligation at beginning of year		$ (169.7)	(140.6)
Service cost		2.0	1.4	1.0
Interest cost		6.6	6.6	5.7
Actuarial gain (loss)		(2.7)	(26.0)
Participant contributions		(6.4)	(6.6)
Benefits paid		12.7	12.2
Other changes in benefit obligation		9.0	(0.7)
Benefit obligation at end of year	(165.7)	(165.7)	(169.7)	(140.6)
Change in plan assets
Fair value of plan assets at beginning of year		639.7	613.0
Actual return on plan assets		(6.9)	31.9
Employer contribution		0.5	0.4
Participant contributions		(6.4)	(6.6)
Benefits paid		12.7	12.2
Fair value of plan assets at end of year	627.0	627.0	639.7	613.0
Amount recognized in statement of financial position
Amount recognized in other assets	461.9	461.9	470.7
Amount recognized in other liabilities	(0.6)	(0.6)	(0.7)
Total assets (liabilities) recognized in statement of financial position	461.3	461.3	470.0
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	(8.6)	(8.6)	(53.0)
Prior service (benefit) cost	(33.0)	(33.0)	(35.9)
Pre-tax accumulated other comprehensive (income) loss	(41.6)	(41.6)	(88.9)
Additional defined benefit plan disclosures
Medicare subsidies received, accrued and included in service cost		0.7	0.7	$ 0.8
Reduction of accumulated postretirement benefit obligation due to change in post-65 medical plan		15.5
Adjustment to accumulated post-retirement benefit obligation related to dental plan benefits	5.8
Information for Defined Benefit Plans With an Accumulated Benefit Obligation in Excess of Plan Assets
Accumulated benefit obligation for defined benefit plans with an accumulated benefit obligation in excess of plan assets	1.3	1.3	1.5
Fair value of plan assets for defined benefit plans with an accumulated benefit obligation in excess of plan assets	$ 0.7	0.7	0.8
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss		43.5	26.7
Amortization of net gain (loss)		0.8	3.4
Amortization of prior service benefit (cost)		18.5	20.3
Plan changes		(15.5)
Total recognized in pre-tax accumulated other comprehensive (income) loss		47.3	50.4
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss		8.5	$ 2.1
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		0.2
Estimated prior service cost (benefit) that will be amortized from accumulated other comprehensive income into net periodic benefit cost during the next fiscal year		$ (20.3)
Retiree health benefits
Components of Net Periodic Benefit Cost (Income)
Participant contributions as a percent of expected cost of coverage for employees hired prior to January 1, 2002, who retired on or after January 1, 2011 (as a percent)	60.00%	60.00%