Income Taxes - Narrative - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
Statutory rate		35.00%	35.00%	35.00%
Effect of tax cuts and jobs act - Deferred federal		$ (476.2)	$ 0.0	$ 0.0
Income before income taxes for U.S. operations		376.1	573.1	1,241.1
Income before income taxes for foreign operations		(4.1)	(6.3)	10.9
Net operating loss carryforwards		18.9
Remaining state loss carryforwards		7.8
Additions for tax positions related to the current year		0.0	0.0	5.5
Additions for tax positions of prior years		4.7	1.0	0.0
Reduction in tax positions of prior years		0.0	26.6	0.7
Unrecognized tax benefits including interest and penalties that would affect the Company's effective tax rate if recognized		10.2	13.1
Total accrued interest and penalties related to uncertain tax positions		4.1	8.9
Increase (decrease) in income tax expense related to interest and penalties on unrecognized tax benefits		(4.8)	$ (3.5)	$ 0.8
Tax Adjustments, Settlements, and Unusual Provisions		5.8
Settlement with taxing authority and lapse in statute of limitations
Income Tax Contingency [Line Items]
Reasonably possible decrease in unrecognized federal and state tax benefits within twelve months		$ 5.2
Subsequent event
Income Tax Contingency [Line Items]
Statutory rate	21.00%