Employee and Agent Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension benefits
Defined benefit plan, fair value of plan assets
Fair value of plan assets	$ 3,373.5	$ 2,926.0	$ 2,498.2
Pension benefits | Qualified benefit plans
Defined benefit plan, fair value of plan assets
Fair value of plan assets	3,373.5	2,926.0
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	712.2	509.7
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	121.6	90.9
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	103.7	78.5
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	459.6	328.6
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	195.1	193.5
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	297.5	485.9
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	28.0	26.1
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	1,223.1	1,009.1
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	13.6	32.7
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	6.4	7.1
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	155.8	129.0
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	55.1	31.5
Pension benefits | Qualified benefit plans | Single client separate account investments | Other
Defined benefit plan, fair value of plan assets
Fair value of plan assets	1.8	3.4
Pension benefits | Qualified benefit plans | Amount measured at net asset value
Defined benefit plan, fair value of plan assets
Fair value of plan assets	155.8	129.0
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	155.8	129.0
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 1
Defined benefit plan, fair value of plan assets
Fair value of plan assets	297.5	485.9
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	297.5	485.9
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 2
Defined benefit plan, fair value of plan assets
Fair value of plan assets	2,920.2	2,311.1
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	712.2	509.7
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	121.6	90.9
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	103.7	78.5
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	459.6	328.6
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	195.1	193.5
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	28.0	26.1
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	1,223.1	1,009.1
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	13.6	32.7
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	6.4	7.1
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	55.1	31.5
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	1.8	3.4
Other postretirement benefits
Defined benefit plan, fair value of plan assets
Fair value of plan assets	780.6	732.8	$ 643.9
Fair value of plan assets included in trust owned life insurance contract	117.6	101.8
Other postretirement benefits | Cash and cash equivalents
Defined benefit plan, fair value of plan assets
Fair value of plan assets	0.5	0.6
Other postretirement benefits | Fixed income investment funds
Defined benefit plan, fair value of plan assets
Fair value of plan assets	625.2	370.5
Other postretirement benefits | U.S. equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	103.5	254.5
Other postretirement benefits | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	51.4	107.2
Other postretirement benefits | Fair value hierarchy Level 1
Defined benefit plan, fair value of plan assets
Fair value of plan assets	663.0	630.8
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	0.5	0.6
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	605.9	331.0
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	38.2	210.3
Other postretirement benefits | Fair value hierarchy Level 1 | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	18.4	88.9
Other postretirement benefits | Fair value hierarchy Level 2
Defined benefit plan, fair value of plan assets
Fair value of plan assets	117.6	102.0
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	19.3	39.5
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	65.3	44.2
Other postretirement benefits | Fair value hierarchy Level 2 | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	$ 33.0	$ 18.3