Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator:
Net income attributable to Ryerson Holding Corporation	$ 25.3	$ 16.4
Denominator:
Weighted average shares outstanding	38,117,397	37,783,761
Dilutive effect of stock-based awards	523,052	441,568
Weighted average shares outstanding adjusted for dilutive securities	38,640,449	38,225,329
Earnings per share
Basic	$ 0.66	$ 0.43
Diluted	$ 0.66	$ 0.43