Income Taxes - Reconciliation of Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Provision (benefit) for federal income taxes at statutory rate		$ 102.0	$ 104.7	$ (137.0)
State income tax expense (benefit), net of federal tax provision		5.5	4.9	(10.2)
Income tax (benefit) expense attributable to non-controlling interest		(7.3)	(12.0)	(7.1)
Tax credits and incentives		(8.3)	(1.6)	(9.7)
Intra-entity basis differences not expected to be realized		5.5	0.0	0.0
Dividends received deduction		0.0	(2.8)	(5.7)
Executive compensation limitation		1.7	1.5	0.3
Stock compensation		(2.2)	0.0	0.0
Amortization - prepaid taxes		0.0	(2.4)	(3.5)
Reversal of deferred taxes related to equity method investment in Alon		0.0	45.3	0.0
Impact of Tax Cuts and Jobs Act	$ 166.9	(0.6)	(166.9)	0.0
Goodwill write-down		5.3	0.0	0.0
Other items		0.3	0.1	1.4
Income tax benefit related to continuing operations		$ 101.9	$ (29.2)	$ (171.5)