Employee Benefit Plans (Summary of Pension Plan Assets Measured at Fair Value) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 178.8	$ 164.9
Interest of lease		26.00%
Quoted prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	113.3	$ 108.4
Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12.3	11.0
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2.8	3.2
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	29.2	35.9
Equity Securities | Quoted prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	29.2	35.9
Equity Securities | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Equity Securities | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Equity Securities at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	29.6	25.0
Equity Securities at NAV | Quoted prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Equity Securities at NAV | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Equity Securities at NAV | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Mutual Funds - Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	59.0	35.8
Mutual Funds - Equity Securities | Quoted prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	59.0	35.8
Mutual Funds - Equity Securities | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Mutual Funds - Equity Securities | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Mutual Funds - Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17.9	33.6
Mutual Funds - Fixed Income | Quoted prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17.9	33.6
Mutual Funds - Fixed Income | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Mutual Funds - Fixed Income | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Bond Funds - Fixed Income at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20.1	16.5
Bond Funds - Fixed Income at NAV | Quoted prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Bond Funds - Fixed Income at NAV | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Bond Funds - Fixed Income at NAV | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Equitable Contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12.3	11.0
Equitable Contract | Quoted prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Equitable Contract | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12.3	11.0
Equitable Contract | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Equitable Contract at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.7	0.8
Equitable Contract at NAV | Quoted prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Equitable Contract at NAV | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Equitable Contract at NAV | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Coal Lease
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2.8	3.2
Coal Lease | Quoted prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Coal Lease | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Coal Lease | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2.8	3.2	$ 3.5
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.2	3.1
Cash and cash equivalents | Quoted prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.2	3.1
Cash and cash equivalents | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Cash and cash equivalents | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0.0	$ 0.0