Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net income attributable to Ryerson Holding Corporation	$ 16.4	$ 17.5	$ 45.9	$ 27.9
Denominator:
Weighted average shares outstanding	37,769,001	37,294,982	37,610,108	37,252,020
Dilutive effect of stock-based awards	149,338	330,769	270,176	333,151
Weighted average shares outstanding adjusted for dilutive securities	37,918,339	37,625,751	37,880,284	37,585,171
Earnings per share
Basic	$ 0.43	$ 0.47	$ 1.22	$ 0.75
Diluted	$ 0.43	$ 0.46	$ 1.21	$ 0.74