Employee and Agent Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pension benefits
Defined benefit plan, fair value of plan assets
Fair value of plan assets	$ 2,909.6	$ 2,726.3	$ 3,550.1
Defined Benefit Plan, Tax Status [Extensible Enumeration]	Qualified benefit plans	Qualified benefit plans
Pension benefits | Pooled separate account investments | Fixed income security portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	$ 27.9
Pension benefits | Pooled separate account investments | U.S. large cap equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	290.9	$ 282.5
Pension benefits | Pooled separate account investments | U.S. small/mid cap equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	49.5	47.3
Pension benefits | Pooled separate account investments | Balanced asset allocation portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	97.0	101.4
Pension benefits | Pooled separate account investments | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	141.8	143.9
Pension benefits | Pooled separate account investments | Real estate investment portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	204.1	238.4
Pension benefits | Single client separate account investments | U.S. government and agencies
Defined benefit plan, fair value of plan assets
Fair value of plan assets	124.9	447.0
Pension benefits | Single client separate account investments | States and political subdivisions
Defined benefit plan, fair value of plan assets
Fair value of plan assets	27.0	25.0
Pension benefits | Single client separate account investments | Corporate debt securities
Defined benefit plan, fair value of plan assets
Fair value of plan assets	1,394.2	1,336.9
Pension benefits | Single client separate account investments | Commercial mortgage-backed securities
Defined benefit plan, fair value of plan assets
Fair value of plan assets		16.1
Pension benefits | Single client separate account investments | Other debt obligations
Defined benefit plan, fair value of plan assets
Fair value of plan assets		4.1
Pension benefits | Single client separate account investments | Hedge funds
Defined benefit plan, fair value of plan assets
Fair value of plan assets		27.9
Pension benefits | Single client separate account investments | Pooled separate account investment
Defined benefit plan, fair value of plan assets
Fair value of plan assets	497.9	43.7
Pension benefits | Single client separate account investments | Other
Defined benefit plan, fair value of plan assets
Fair value of plan assets	54.4	12.1
Pension benefits | Amount measured at net asset value
Defined benefit plan, fair value of plan assets
Fair value of plan assets		27.9
Pension benefits | 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		27.9
Pension benefits | Fair value hierarchy Level 1
Defined benefit plan, fair value of plan assets
Fair value of plan assets	124.9	447.0
Pension benefits | 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	124.9	447.0
Pension benefits | Fair value hierarchy Level 2
Defined benefit plan, fair value of plan assets
Fair value of plan assets	2,784.7	2,251.4
Pension benefits | Fair value hierarchy Level 2 | Pooled separate account investments | Fixed income security portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	27.9
Pension benefits | 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	290.9	282.5
Pension benefits | 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	49.5	47.3
Pension benefits | 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	97.0	101.4
Pension benefits | 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	141.8	143.9
Pension benefits | 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	204.1	238.4
Pension benefits | 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	27.0	25.0
Pension benefits | 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	1,394.2	1,336.9
Pension benefits | 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		16.1
Pension benefits | 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		4.1
Pension benefits | 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	497.9	43.7
Pension benefits | Fair value hierarchy Level 2 | Single client separate account investments | Other
Defined benefit plan, fair value of plan assets
Fair value of plan assets	54.4	12.1
Other postretirement benefits
Defined benefit plan, fair value of plan assets
Fair value of plan assets	99.8	94.4	$ 119.8
Other postretirement benefits | Cash and cash equivalents
Defined benefit plan, fair value of plan assets
Fair value of plan assets	1.3	0.6
Other postretirement benefits | Fixed income security portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	47.9	47.6
Other postretirement benefits | U.S. equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	36.7	33.6
Other postretirement benefits | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	13.9	12.6
Other postretirement benefits | Fair value hierarchy Level 1
Defined benefit plan, fair value of plan assets
Fair value of plan assets	99.8	94.4
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	1.3	0.6
Other postretirement benefits | Fair value hierarchy Level 1 | Fixed income security portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	47.9	47.6
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	36.7	33.6
Other postretirement benefits | Fair value hierarchy Level 1 | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	$ 13.9	$ 12.6