Employee and Agent Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension benefits
Defined benefit plan, fair value of plan assets
Fair value of plan assets	$ 2,542.2	$ 2,190.8	$ 2,166.3
Pension benefits | Qualified benefit plans
Defined benefit plan, fair value of plan assets
Fair value of plan assets	2,542.2	2,190.8
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	651.2	772.7
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	112.2	153.0
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	128.4	123.0
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	325.8	294.3
Pension benefits | Qualified benefit plans | Pooled separate account investments | Fixed income security portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets		13.4
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	126.2	129.4
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	292.7	226.9
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	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	745.2	444.8
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	21.3	17.8
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	3.4	2.7
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	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	4.6	6.9
Pension benefits | Qualified benefit plans | Single client separate account investments | Other
Defined benefit plan, fair value of plan assets
Fair value of plan assets	(6.1)	5.9
Pension benefits | Qualified benefit plans | Amount measured at net asset value
Defined benefit plan, fair value of plan assets
Fair value of plan assets	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	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	293.5	226.9
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	292.7	226.9
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	0.8
Pension benefits | Qualified benefit plans | Fair value hierarchy Level 2
Defined benefit plan, fair value of plan assets
Fair value of plan assets	2,128.3	1,963.9
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	651.2	772.7
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	112.2	153.0
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	128.4	123.0
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	325.8	294.3
Pension benefits | Qualified benefit plans | 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		13.4
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	126.2	129.4
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	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	745.2	444.8
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	21.3	17.8
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	3.4	2.7
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	4.6	6.9
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	(6.9)	5.9
Other postretirement benefits
Defined benefit plan, fair value of plan assets
Fair value of plan assets	696.8	628.6	$ 627.0
Fair value of plan assets included in trust owned life insurance contract	93.2	81.8
Other postretirement benefits | Cash and cash equivalents
Defined benefit plan, fair value of plan assets
Fair value of plan assets	0.3	1.2
Other postretirement benefits | U.S. equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	159.4	145.5
Other postretirement benefits | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	67.8	51.2
Other postretirement benefits | Fixed income investment funds
Defined benefit plan, fair value of plan assets
Fair value of plan assets	209.8	195.5
Other postretirement benefits | Alternative mutual fund portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	251.4	227.5
Other postretirement benefits | Real estate mutual fund
Defined benefit plan, fair value of plan assets
Fair value of plan assets	8.1	7.7
Other postretirement benefits | Fair value hierarchy Level 1
Defined benefit plan, fair value of plan assets
Fair value of plan assets	603.7	547.1
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	1.2
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	108.5	103.2
Other postretirement benefits | Fair value hierarchy Level 1 | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	57.7	43.2
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	177.7	164.3
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	251.4	227.5
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	8.1	7.7
Other postretirement benefits | Fair value hierarchy Level 2
Defined benefit plan, fair value of plan assets
Fair value of plan assets	93.1	81.5
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	50.9	42.3
Other postretirement benefits | Fair value hierarchy Level 2 | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	10.1	8.0
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	$ 32.1	$ 31.2