Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Millions	Total		Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Retained Earnings	Treasury Stock	Non-Controlling Interest in Subsidiaries	Commodity contracts	Commodity contracts Accumulated Other Comprehensive Income	Interest rate contracts	Interest rate contracts Accumulated Other Comprehensive Income
Beginning balance (shares) at Dec. 31, 2017			81,533,548				(762,623)
Beginning balance at Dec. 31, 2017	$ 1,964.2		$ 0.8	$ 900.1	$ 6.9	$ 767.8	$ (25.0)	$ 313.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification for stranded tax effects resulting from the the Tax Reform Act	1.6
Beginning balance (shares) at Dec. 31, 2017			81,533,548				(762,623)
Beginning balance at Dec. 31, 2017	1,964.2		$ 0.8	900.1	6.9	767.8	$ (25.0)	313.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(25.5)	[1]				(40.4)		14.9
Other comprehensive income related to commodity contracts or interest rate contracts	2.1				2.1				$ 2.1		$ (0.4)	$ (0.4)
Other comprehensive income related to postretirement benefit plans	0.1				0.1
Foreign currency translation gain (loss)	(0.4)				(0.4)
Common stock dividends	(17.0)					(17.0)
Distribution to non-controlling interest	(6.9)							(6.9)
Equity-based compensation expense	4.8			4.6				0.2
Issuance of stock for non-controlling interest repurchase, net of tax (in shares)			5,649,373
Issuance of stock for non-controlling interest repurchase, net of tax	13.5		$ 0.1	140.4				(127.0)
De-recognition of non-controlling interest	$ (18.7)							(18.7)
Reclassification for stranded tax effects resulting from the the Tax Reform Act					1.6	(1.6)
Repurchase of common stock (shares)	(2,600,000)						(2,569,932)
Repurchase of common stock	$ (95.3)						$ (95.3)
Taxes due to the net settlement of equity-based compensation	(1.8)			(1.8)
Exercise of equity-based awards (shares)			122,747
Other					0.1	(0.1)
Ending balance at Mar. 31, 2018	1,774.3		$ 0.9	1,043.3	10.0	664.3	$ (120.3)	176.1
Ending balance (shares) at Mar. 31, 2018			87,305,668				(3,332,555)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adopting accounting principle regarding income tax effect of intra-equity transfers	(44.4)					(44.4)
Beginning balance (shares) at Dec. 31, 2018			90,478,075				(12,477,780)
Beginning balance at Dec. 31, 2018	1,808.1		$ 0.9	1,135.4	28.6	981.8	$ (514.1)	175.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	154.4					149.3		5.1
Other comprehensive income related to commodity contracts or interest rate contracts	10.3								$ 10.3	$ 10.3	$ 0.0
Other comprehensive income related to postretirement benefit plans	0.2				0.2
Foreign currency translation gain (loss)	0.2				0.2			0.0
Common stock dividends	(21.0)					(21.0)		0.0
Distribution to non-controlling interest	(7.7)							(7.7)
Equity-based compensation expense	$ 5.0			4.9				0.1
Repurchase of common stock (shares)	(1,300,000)						(1,291,644)
Repurchase of common stock	$ (46.2)						$ (46.2)	0.0
Taxes due to the net settlement of equity-based compensation	(4.5)			(4.5)				0.0
Exercise of equity-based awards (shares)			244,566
Other	(0.3)			(0.3)			0.0
Ending balance at Mar. 31, 2019	$ 1,898.5		$ 0.9	$ 1,135.5	$ 39.3	$ 1,110.1	$ (560.3)	$ 173.0
Ending balance (shares) at Mar. 31, 2019			90,722,641				(13,769,424)

[1] Income tax benefit for the quarter ended March 31, 2018 reflects a correction made in our 2018 Annual Report on Form 10-K (filed on March 1, 2019) to record additional deferred tax expense totaling $5.5 million related to the recognition of a valuation allowance on deferred tax assets recognized in connection with the Big Spring Logistic Assets Acquisition (see Note 5) not previously reported in our March 31, 2018 Quarterly Report on Form 10-Q filed on May 10, 2018. Such amount is not considered material to the financial statements or the trend of earnings for that period. See Note 23 to our annual audited consolidated financial statements included in Part II, Item 8 of our 2018 Annual Report on Form 10-K filed on March 1, 2019 for further discussion.