Employee and Agent Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Pension benefits
Defined benefit plan, fair value of plan assets
Fair value of plan assets	$ 2,813.7	$ 2,909.6	$ 2,726.3
Defined Benefit Plan, Tax Status [Extensible Enumeration]	Qualified Plan [Member]	Qualified Plan [Member]
Pension benefits \| Pooled separate account investments \| Fixed income security portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	$ 26.8	$ 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	236.5	290.9
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	98.4	49.5
Pension benefits \| Pooled separate account investments \| Balanced asset allocation portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	135.0	97.0
Pension benefits \| Pooled separate account investments \| International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	135.9	141.8
Pension benefits \| Pooled separate account investments \| Real estate investment portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	133.6	204.1
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	93.7	124.9
Pension benefits \| Single client separate account investments \| States and political subdivisions
Defined benefit plan, fair value of plan assets
Fair value of plan assets	15.5	27.0
Pension benefits \| Single client separate account investments \| Corporate debt securities
Defined benefit plan, fair value of plan assets
Fair value of plan assets	1,387.2	1,394.2
Pension benefits \| Single client separate account investments \| Hedge funds
Defined benefit plan, fair value of plan assets
Fair value of plan assets	3.6
Pension benefits \| Single client separate account investments \| Mutual funds
Defined benefit plan, fair value of plan assets
Fair value of plan assets	109.9
Pension benefits \| Single client separate account investments \| Pooled separate account investment
Defined benefit plan, fair value of plan assets
Fair value of plan assets	423.0	497.9
Pension benefits \| Single client separate account investments \| Other
Defined benefit plan, fair value of plan assets
Fair value of plan assets	14.6	54.4
Pension benefits \| Amount measured at net asset value
Defined benefit plan, fair value of plan assets
Fair value of plan assets	3.6
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	3.6
Pension benefits \| Fair value hierarchy Level 1
Defined benefit plan, fair value of plan assets
Fair value of plan assets	203.6	124.9
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	93.7	124.9
Pension benefits \| Fair value hierarchy Level 1 \| Single client separate account investments \| Mutual funds
Defined benefit plan, fair value of plan assets
Fair value of plan assets	109.9
Pension benefits \| Fair value hierarchy Level 2
Defined benefit plan, fair value of plan assets
Fair value of plan assets	2,606.5	2,784.7
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	26.8	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	236.5	290.9
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	98.4	49.5
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	135.0	97.0
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	135.9	141.8
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	133.6	204.1
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	15.5	27.0
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	1,387.2	1,394.2
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	423.0	497.9
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	14.6	54.4
Other postretirement benefits
Defined benefit plan, fair value of plan assets
Fair value of plan assets	102.0	99.8	$ 94.4
Other postretirement benefits \| Cash and cash equivalents
Defined benefit plan, fair value of plan assets
Fair value of plan assets	1.0	1.3
Other postretirement benefits \| Fixed income security portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	48.6	47.9
Other postretirement benefits \| U.S. equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	38.7	36.7
Other postretirement benefits \| International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	13.7	13.9
Other postretirement benefits \| Fair value hierarchy Level 1
Defined benefit plan, fair value of plan assets
Fair value of plan assets	102.0	99.8
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	1.0	1.3
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	48.6	47.9
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	38.7	36.7
Other postretirement benefits \| Fair value hierarchy Level 1 \| International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	$ 13.7	$ 13.9