Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Numerator:
Net income attributable to Ryerson Holding Corporation	$ 16.4	$ 29.5
Denominator:
Weighted average shares outstanding	37,783,761	37,449,450
Dilutive effect of stock-based awards	441,568	391,013
Weighted average shares outstanding adjusted for dilutive securities	38,225,329	37,840,463
Earnings per share
Basic	$ 0.43	$ 0.79
Diluted	$ 0.43	$ 0.78