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
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 (loss) income	154.4				149.3		5.1
Other comprehensive income related to commodity contracts, net								$ 10.3	$ 10.3
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)	(1,291,644)					(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 (shares)	0
Other	$ (0.3)		(0.3)	0.0	0.0	0.0	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)
Beginning balance (shares) at Dec. 31, 2019		90,987,025				(17,516,814)
Beginning balance at Dec. 31, 2019	1,835.3	$ 0.9	1,151.9	0.1	1,205.6	$ (692.2)	169.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	(307.0)				(314.4)		7.4
Other comprehensive income related to commodity contracts, net								$ 1.4	$ 1.4
Other comprehensive income related to postretirement benefit plans	0.0
Foreign currency translation gain (loss)	(0.3)			(0.3)			0.0
Common stock dividends	(23.1)				(23.1)		0.0
Distribution to non-controlling interest	(8.6)						(8.6)
Equity-based compensation expense	$ 6.3		6.2				0.1
Repurchase of common stock (shares)	(58,713)					(58,713)
Repurchase of common stock	$ (1.9)						0.0
Repurchase of Non-Controlling Interest	(5.0)						(5.0)
Taxes due to the net settlement of equity-based compensation	$ (0.7)		(0.7)				0.0
Exercise of equity-based awards (shares)	102,895	102,895
Ending balance at Mar. 31, 2020	$ 1,489.9	$ 0.9	$ 1,157.4	$ 1.2	$ 861.6	$ (694.1)	$ 162.9
Ending balance (shares) at Mar. 31, 2020		91,089,920				(17,575,527)