Retirement and Postretirement Plans (Qualified Plan's Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 13.0	$ 139.8	$ 146.0
Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 12.8	$ 139.4
Asset allocations (as a percent)	100.00%	100.00%
Qualified Plan [Member] | Level 1 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 12.8	$ 139.4
Qualified Plan [Member] | Level 2 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0.0	0.0
Qualified Plan [Member] | Level 3 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 0.0	$ 0.0
Qualified Plan [Member] | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Asset allocations (as a percent)	1.50%	5.00%
Qualified Plan [Member] | Fixed maturity investments
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 0.0	$ 132.4
Asset allocations (as a percent)	0.00%	95.00%
Qualified Plan [Member] | Fixed maturity investments | Level 1 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 0.0	$ 132.4
Qualified Plan [Member] | Fixed maturity investments | Level 2 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0.0	0.0
Qualified Plan [Member] | Fixed maturity investments | Level 3 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0.0	0.0
Qualified Plan [Member] | Common equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 12.8	$ 0.0
Asset allocations (as a percent)	98.50%	0.00%
Qualified Plan [Member] | Common equity securities | Level 1 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 12.8	$ 0.0
Qualified Plan [Member] | Common equity securities | Level 2 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0.0	0.0
Qualified Plan [Member] | Common equity securities | Level 3 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0.0	0.0
Qualified Plan [Member] | Short-term investments
Defined Benefit Plan Disclosure [Line Items]
Fair value	0.0	7.0
Qualified Plan [Member] | Short-term investments | Level 1 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0.0	7.0
Qualified Plan [Member] | Short-term investments | Level 2 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	0.0	0.0
Qualified Plan [Member] | Short-term investments | Level 3 Inputs
Defined Benefit Plan Disclosure [Line Items]
Fair value	$ 0.0	$ 0.0