Employee and Agent Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension benefits
Defined benefit plan, fair value of plan assets
Fair value of plan assets	$ 2,926.0	$ 2,498.2	$ 2,542.2
Pension benefits | Qualified benefit plans
Defined benefit plan, fair value of plan assets
Fair value of plan assets	2,926.0	2,498.2
Pension benefits | Qualified benefit plans | Pooled separate account investments | U.S. large cap equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	509.7	387.7
Pension benefits | Qualified benefit plans | Pooled separate account investments | U.S. small/mid cap equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	90.9	64.4
Pension benefits | Qualified benefit plans | Pooled separate account investments | Balanced asset allocation portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	78.5	116.3
Pension benefits | Qualified benefit plans | Pooled separate account investments | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	328.6	220.9
Pension benefits | Qualified benefit plans | Pooled separate account investments | Real estate investment portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	193.5	133.6
Pension benefits | Qualified benefit plans | Single client separate account investments | U.S. government and agencies
Defined benefit plan, fair value of plan assets
Fair value of plan assets	485.9	418.8
Pension benefits | Qualified benefit plans | Single client separate account investments | States and political subdivisions
Defined benefit plan, fair value of plan assets
Fair value of plan assets	26.1	24.0
Pension benefits | Qualified benefit plans | Single client separate account investments | Corporate debt securities
Defined benefit plan, fair value of plan assets
Fair value of plan assets	1,009.1	934.5
Pension benefits | Qualified benefit plans | Single client separate account investments | Commercial mortgage-backed securities
Defined benefit plan, fair value of plan assets
Fair value of plan assets	32.7	30.7
Pension benefits | Qualified benefit plans | Single client separate account investments | Other debt obligations
Defined benefit plan, fair value of plan assets
Fair value of plan assets	7.1	8.0
Pension benefits | Qualified benefit plans | Single client separate account investments | Hedge funds
Defined benefit plan, fair value of plan assets
Fair value of plan assets	129.0	124.7
Pension benefits | Qualified benefit plans | Single client separate account investments | Pooled separate account investment
Defined benefit plan, fair value of plan assets
Fair value of plan assets	31.5	23.9
Pension benefits | Qualified benefit plans | Single client separate account investments | Other
Defined benefit plan, fair value of plan assets
Fair value of plan assets	3.4	10.7
Pension benefits | Qualified benefit plans | Amount measured at net asset value
Defined benefit plan, fair value of plan assets
Fair value of plan assets	129.0	124.7
Pension benefits | Qualified benefit plans | Amount measured at net asset value | Single client separate account investments | Hedge funds
Defined benefit plan, fair value of plan assets
Fair value of plan assets	129.0	124.7
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 1
Defined benefit plan, fair value of plan assets
Fair value of plan assets	485.9	419.1
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 1 | Single client separate account investments | U.S. government and agencies
Defined benefit plan, fair value of plan assets
Fair value of plan assets	485.9	418.8
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 1 | Single client separate account investments | Other
Defined benefit plan, fair value of plan assets
Fair value of plan assets		0.3
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 2
Defined benefit plan, fair value of plan assets
Fair value of plan assets	2,311.1	1,954.4
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 2 | Pooled separate account investments | U.S. large cap equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	509.7	387.7
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 2 | Pooled separate account investments | U.S. small/mid cap equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	90.9	64.4
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 2 | Pooled separate account investments | Balanced asset allocation portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	78.5	116.3
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 2 | Pooled separate account investments | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	328.6	220.9
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 2 | Pooled separate account investments | Real estate investment portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	193.5	133.6
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 2 | Single client separate account investments | States and political subdivisions
Defined benefit plan, fair value of plan assets
Fair value of plan assets	26.1	24.0
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 2 | Single client separate account investments | Corporate debt securities
Defined benefit plan, fair value of plan assets
Fair value of plan assets	1,009.1	934.5
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 2 | Single client separate account investments | Commercial mortgage-backed securities
Defined benefit plan, fair value of plan assets
Fair value of plan assets	32.7	30.7
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 2 | Single client separate account investments | Other debt obligations
Defined benefit plan, fair value of plan assets
Fair value of plan assets	7.1	8.0
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 2 | Single client separate account investments | Pooled separate account investment
Defined benefit plan, fair value of plan assets
Fair value of plan assets	31.5	23.9
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 2 | Single client separate account investments | Other
Defined benefit plan, fair value of plan assets
Fair value of plan assets	3.4	10.4
Other postretirement benefits
Defined benefit plan, fair value of plan assets
Fair value of plan assets	732.8	643.9	$ 696.8
Fair value of plan assets included in trust owned life insurance contract	101.8	86.1
Other postretirement benefits | Cash and cash equivalents
Defined benefit plan, fair value of plan assets
Fair value of plan assets	0.6	0.3
Other postretirement benefits | Fixed income investment funds
Defined benefit plan, fair value of plan assets
Fair value of plan assets	370.5	203.7
Other postretirement benefits | U.S. equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	254.5	135.8
Other postretirement benefits | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	107.2	61.2
Other postretirement benefits | Alternative mutual fund portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets		235.4
Other postretirement benefits | Real estate mutual fund
Defined benefit plan, fair value of plan assets
Fair value of plan assets		7.5
Other postretirement benefits | Fair value hierarchy Level 1
Defined benefit plan, fair value of plan assets
Fair value of plan assets	630.8	557.8
Other postretirement benefits | Fair value hierarchy Level 1 | Cash and cash equivalents
Defined benefit plan, fair value of plan assets
Fair value of plan assets	0.6	0.3
Other postretirement benefits | Fair value hierarchy Level 1 | Fixed income investment funds
Defined benefit plan, fair value of plan assets
Fair value of plan assets	331.0	166.2
Other postretirement benefits | Fair value hierarchy Level 1 | U.S. equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	210.3	101.0
Other postretirement benefits | Fair value hierarchy Level 1 | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	88.9	47.4
Other postretirement benefits | Fair value hierarchy Level 1 | Alternative mutual fund portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets		235.4
Other postretirement benefits | Fair value hierarchy Level 1 | Real estate mutual fund
Defined benefit plan, fair value of plan assets
Fair value of plan assets		7.5
Other postretirement benefits | Fair value hierarchy Level 2
Defined benefit plan, fair value of plan assets
Fair value of plan assets	102.0	86.1
Other postretirement benefits | Fair value hierarchy Level 2 | Fixed income investment funds
Defined benefit plan, fair value of plan assets
Fair value of plan assets	39.5	37.5
Other postretirement benefits | Fair value hierarchy Level 2 | U.S. equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	44.2	34.8
Other postretirement benefits | Fair value hierarchy Level 2 | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	$ 18.3	$ 13.8