Employee Benefit Plans US Pension Assets, Fair Value Measurement by Input Level (Details) - Domestic Plan - Pension Plan - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	$ 1,575.0	$ 1,449.2
Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	325.7	299.2
Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	353.7	316.7
Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
NAV as a Practical Expedient
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	895.6	833.3
Equity Securities | U.S. Large Cap
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	103.3	85.5
Equity Securities | U.S. Large Cap | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Equity Securities | U.S. Large Cap | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Equity Securities | U.S. Large Cap | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Equity Securities | U.S. Large Cap | NAV as a Practical Expedient
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	103.3	85.5
Equity Securities | U.S. Small Cap
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	45.2	50.2
Equity Securities | U.S. Small Cap | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	45.2	50.2
Equity Securities | U.S. Small Cap | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Equity Securities | U.S. Small Cap | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Equity Securities | U.S. Small Cap | NAV as a Practical Expedient
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Equity Securities | Global Equity Securities
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	325.9	259.7
Equity Securities | Global Equity Securities | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Equity Securities | Global Equity Securities | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Equity Securities | Global Equity Securities | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Equity Securities | Global Equity Securities | NAV as a Practical Expedient
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	325.9	259.7
Equity Securities | International Securities
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	35.7	26.3
Equity Securities | International Securities | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Equity Securities | International Securities | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Equity Securities | International Securities | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Equity Securities | International Securities | NAV as a Practical Expedient
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	35.7	26.3
Equity Securities | Emerging Markets
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	61.5	49.6
Equity Securities | Emerging Markets | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Equity Securities | Emerging Markets | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Equity Securities | Emerging Markets | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Equity Securities | Emerging Markets | NAV as a Practical Expedient
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	61.5	49.6
United States Government and Government Agencies and Authorities
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	260.4	238.9
United States Government and Government Agencies and Authorities | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	248.5	227.8
United States Government and Government Agencies and Authorities | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	11.9	11.1
United States Government and Government Agencies and Authorities | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
United States Government and Government Agencies and Authorities | NAV as a Practical Expedient
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Corporate Debt Securities
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	335.5	303.6
Corporate Debt Securities | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Corporate Debt Securities | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	335.5	303.6
Corporate Debt Securities | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Corporate Debt Securities | NAV as a Practical Expedient
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
States, Municipalities, and Political Subdivisions
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	6.3	2.0
States, Municipalities, and Political Subdivisions | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
States, Municipalities, and Political Subdivisions | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	6.3	2.0
States, Municipalities, and Political Subdivisions | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
States, Municipalities, and Political Subdivisions | NAV as a Practical Expedient
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Opportunistic Credits
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	193.3	181.8
Opportunistic Credits | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Opportunistic Credits | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Opportunistic Credits | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Opportunistic Credits | NAV as a Practical Expedient
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	193.3	181.8
Real Estate
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	96.1	88.9
Real Estate | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Real Estate | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Real Estate | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Real Estate | NAV as a Practical Expedient
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	96.1	88.9
Private Equity Direct Investments
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	45.1	37.4
Private Equity Direct Investments | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Private Equity Direct Investments | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Private Equity Direct Investments | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Private Equity Direct Investments | NAV as a Practical Expedient
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	45.1	37.4
Private Equity Funds of Funds
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	34.5	32.0
Private Equity Funds of Funds | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Private Equity Funds of Funds | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Private Equity Funds of Funds | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Private Equity Funds of Funds | NAV as a Practical Expedient
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	34.5	32.0
Hedge Funds of Funds
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.2	72.1
Hedge Funds of Funds | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Hedge Funds of Funds | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Hedge Funds of Funds | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Hedge Funds of Funds | NAV as a Practical Expedient
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.2	72.1
Cash and Cash Equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	32.0	21.2
Cash and Cash Equivalents | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	32.0	21.2
Cash and Cash Equivalents | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Cash and Cash Equivalents | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	0.0	0.0
Cash and Cash Equivalents | NAV as a Practical Expedient
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair Value of Invested Pension Assets	$ 0.0	$ 0.0