Employee and Agent Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension benefits
Defined benefit plan, fair value of plan assets
Fair value of plan assets	$ 2,498.2	$ 2,542.2	$ 2,190.8
Pension benefits | Qualified benefit plans
Defined benefit plan, fair value of plan assets
Fair value of plan assets	2,498.2	2,542.2
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	387.7	651.2
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	64.4	112.2
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	116.3	128.4
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	220.9	325.8
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	133.6	126.2
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	418.8	292.7
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	24.0	16.9
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	934.5	745.2
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	30.7	21.3
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	8.0	3.4
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	124.7	120.4
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	23.9	4.6
Pension benefits | Qualified benefit plans | Single client separate account investments | Other
Defined benefit plan, fair value of plan assets
Fair value of plan assets	10.7	(6.1)
Pension benefits | Qualified benefit plans | Amount measured at net asset value
Defined benefit plan, fair value of plan assets
Fair value of plan assets	124.7	120.4
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	124.7	120.4
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 1
Defined benefit plan, fair value of plan assets
Fair value of plan assets	419.1	293.5
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	418.8	292.7
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	0.8
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 2
Defined benefit plan, fair value of plan assets
Fair value of plan assets	1,954.4	2,128.3
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	387.7	651.2
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	64.4	112.2
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	116.3	128.4
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	220.9	325.8
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	133.6	126.2
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	24.0	16.9
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	934.5	745.2
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	30.7	21.3
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	8.0	3.4
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	23.9	4.6
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	10.4	(6.9)
Other postretirement benefits
Defined benefit plan, fair value of plan assets
Fair value of plan assets	643.9	696.8	$ 628.6
Fair value of plan assets included in trust owned life insurance contract	86.1	93.2
Other postretirement benefits | Cash and cash equivalents
Defined benefit plan, fair value of plan assets
Fair value of plan assets	0.3	0.3
Other postretirement benefits | Fixed income investment funds
Defined benefit plan, fair value of plan assets
Fair value of plan assets	203.7	209.8
Other postretirement benefits | U.S. equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	135.8	159.4
Other postretirement benefits | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	61.2	67.8
Other postretirement benefits | Alternative mutual fund portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	235.4	251.4
Other postretirement benefits | Real estate mutual fund
Defined benefit plan, fair value of plan assets
Fair value of plan assets	7.5	8.1
Other postretirement benefits | Fair value hierarchy Level 1
Defined benefit plan, fair value of plan assets
Fair value of plan assets	557.8	603.7
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	0.3	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	166.2	177.7
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	101.0	108.5
Other postretirement benefits | Fair value hierarchy Level 1 | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	47.4	57.7
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	251.4
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	8.1
Other postretirement benefits | Fair value hierarchy Level 2
Defined benefit plan, fair value of plan assets
Fair value of plan assets	86.1	93.1
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	37.5	32.1
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	34.8	50.9
Other postretirement benefits | Fair value hierarchy Level 2 | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	$ 13.8	$ 10.1