Income Tax Matters, Narrative (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Income Tax Contingency
Income tax provision (benefit)		$ 15.1	$ (175.8)
Effective tax rate		36.50%	37.60%
Effective income tax rate reconciliation, share-based compensation cost, amount		$ 0.7
Effective income tax rate reconciliation, share-based compensation cost, percent		(1.70%)
Effective income tax rate reconciliation, change in deferred tax assets valuation allowance, amount		$ 0.3
Effective income tax rate reconciliation, change in deferred tax assets valuation allowance, percent		0.70%
Gross unrecognized tax benefits		$ 1.7		$ 1.7
Gross unrecognized tax benefits that would impact effective tax rate		0.6		0.6
Cumulative-effect adjustment	[1]			0.7
Accounting Standards Update 2016-09 | New Accounting Pronouncement, Early Adoption, Effect
Income Tax Contingency
Cumulative-effect adjustment	[1]			$ 0.7
Union VEBA
Income Tax Contingency
Income tax benefit on removal of Union VEBA net assets		$ 184.4	$ 184.4

[1] See Note 4 and Note 6 for discussion of our adoption of ASU 2016-09 (as defined in Note 1).