Selected Quarterly Financial Data (Unaudited) - Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended														12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018	Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 446.9	$ 445.0		$ 434.1		$ 410.9		$ 374.4	$ 378.6		$ 353.0		$ 354.4		$ 1,736.9	$ 1,460.4	$ 1,462.4
Operating costs:
Cost of revenues	369.3	356.7		345.7		332.8		310.9	308.9		283.0		285.6		1,404.5	1,188.4	1,167.7
Selling, general, and administrative expenses	47.1	45.5		46.1		40.8		36.8	40.1		39.4		37.6		179.5	153.9	163.0
Impairment charge								0.0	23.2		0.0		0.0		0.0	23.2	0.0
Operating profit	30.5	42.8		42.3		37.3		26.7	6.4		30.6		31.2		152.9	94.9	131.7
Income before income taxes	28.5	41.9		40.8		35.6		28.8	6.6		29.4		30.2		146.8	95.0	130.1
Provision (benefit) for income taxes	7.4	9.2		9.0		7.9		1.1	3.4		6.8		8.0		33.5	19.3	40.4
Net income	$ 21.1	$ 32.7		$ 31.8		$ 27.7		$ 27.7	$ 3.2		$ 22.6		$ 22.2		$ 113.3	$ 75.7	$ 89.7
Net income per common share:
Basic (in dollars per share)	$ 0.44	$ 0.68	[1]	$ 0.66	[1]	$ 0.57	[1]	$ 0.56	$ 0.07	[1]	$ 0.46	[1]	$ 0.45	[1]	$ 2.34	$ 1.55	$ 1.84
Diluted (in dollars per share)	$ 0.43	$ 0.67	[1]	$ 0.65	[1]	$ 0.56	[1]	$ 0.56	$ 0.07	[1]	$ 0.46	[1]	$ 0.45	[1]	$ 2.32	$ 1.54	$ 1.84

[1] For periods prior to the Separation, the denominator for basic and diluted net income per common share was calculated using the 48.8 million shares of common stock outstanding immediately following the Separation.