Consolidated Statements of Stockholder's Equity Consolidated Statements of Stockholder's Equity - USD ($) shares in Thousands, $ in Millions	Total	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning balance (in shares) at Dec. 31, 2018		16,482
Beginning balance at Dec. 31, 2018	$ 426.4	$ 0.0	$ 6.4	$ 487.3	$ (67.3)
Net income	3.8			3.8
Performance share, restricted stock unit, and stock option awards (in shares)		33
APIC, Share-based Payment Arrangement, Increase for Cost Recognition	1.2		1.2
Stock-based compensation expense	(0.4)		(0.4)
Pension and other postretirement employee benefit plans, net of tax	1.2				1.2
Beginning balance (in shares) at Mar. 31, 2019		16,515
Beginning balance at Mar. 31, 2019	432.2	$ 0.0	7.2	491.1	(66.1)
Pension and OPEB, Tax	0.5
Beginning balance (in shares) at Dec. 31, 2018		16,482
Beginning balance at Dec. 31, 2018	426.4	$ 0.0	6.4	487.3	(67.3)
Net income	3.4
Beginning balance (in shares) at Jun. 30, 2019		16,515
Beginning balance at Jun. 30, 2019	434.3	$ 0.0	8.4	490.7	(64.8)
Beginning balance (in shares) at Mar. 31, 2019		16,515
Beginning balance at Mar. 31, 2019	432.2	$ 0.0	7.2	491.1	(66.1)
Net income	(0.4)			(0.4)
APIC, Share-based Payment Arrangement, Increase for Cost Recognition	1.2		1.2
Pension and other postretirement employee benefit plans, net of tax	1.3				1.3
Beginning balance (in shares) at Jun. 30, 2019		16,515
Beginning balance at Jun. 30, 2019	434.3	$ 0.0	8.4	490.7	(64.8)
Pension and OPEB, Tax	0.5
Beginning balance (in shares) at Dec. 31, 2019		16,515
Beginning balance at Dec. 31, 2019	432.0	$ 0.0	9.8	481.7	(59.5)
Net income	10.3			10.3
Performance share, restricted stock unit, and stock option awards (in shares)		54
APIC, Share-based Payment Arrangement, Increase for Cost Recognition	1.3		1.3
Stock-based compensation expense	(0.7)		(0.7)
Pension and other postretirement employee benefit plans, net of tax	1.8				1.8
Beginning balance (in shares) at Mar. 31, 2020		16,569
Beginning balance at Mar. 31, 2020	444.8	$ 0.0	10.4	492.1	(57.7)
Pension and OPEB, Tax	0.6
Beginning balance (in shares) at Dec. 31, 2019		16,515
Beginning balance at Dec. 31, 2019	432.0	$ 0.0	9.8	481.7	(59.5)
Net income	33.1
Beginning balance (in shares) at Jun. 30, 2020		16,571
Beginning balance at Jun. 30, 2020	470.8	$ 0.0	11.8	514.8	(55.8)
Beginning balance (in shares) at Mar. 31, 2020		16,569
Beginning balance at Mar. 31, 2020	444.8	$ 0.0	10.4	492.1	(57.7)
Net income	22.8			22.8
Performance share, restricted stock unit, and stock option awards (in shares)		2
APIC, Share-based Payment Arrangement, Increase for Cost Recognition	1.4		1.4
Stock-based compensation expense	0.0		0.0
Pension and other postretirement employee benefit plans, net of tax	1.8				1.8
Beginning balance (in shares) at Jun. 30, 2020		16,571
Beginning balance at Jun. 30, 2020	470.8	$ 0.0	$ 11.8	$ 514.8	$ (55.8)
Pension and OPEB, Tax	$ 0.6