Background and Nature of Operations - Public Offerings (Detail) $ / shares in Units, $ in Millions								3 Months Ended		6 Months Ended		9 Months Ended
	Mar. 18, 2016 $ / shares shares	Sep. 16, 2015 $ / shares shares	Jun. 03, 2015 $ / shares shares	May 15, 2015 $ / shares shares	Mar. 18, 2015 $ / shares shares	Feb. 10, 2014 USD ($) $ / shares shares	Feb. 03, 2014	Jun. 30, 2016 shares	Jun. 30, 2015 shares	Jun. 30, 2016 shares	Jun. 30, 2015 shares	Sep. 30, 2015 USD ($)
Repurchase from LSF8 in private transaction
Subsidiary, Sale of Stock [Line Items]
Shares repurchased (shares) | shares	900,000	1,007,500		913,200				0	913,200	900,000	913,200
Term Loan Facility | Second Lien Credit Agreement
Subsidiary, Sale of Stock [Line Items]
Repayments of debt through cash on hand						$ 6.1
Repurchased face amount						155.0
Prepayment premium						$ 3.1
Initial Public Offering
Subsidiary, Sale of Stock [Line Items]
Shares issued at public offering (shares) | shares						11,765,000
Offering price per share (usd per share) | $ / shares						$ 14.00
Net proceeds after underwriting discounts and commissions						$ 154.0
One-time payment to Lone Star for termination of asset advisory agreement						2.0
Initial Public Offering | Common Stock
Subsidiary, Sale of Stock [Line Items]
Stock split ratio (shares)							32,304
Initial Public Offering | Term Loan Facility | Second Lien Credit Agreement
Subsidiary, Sale of Stock [Line Items]
Repayment of debt through net proceeds from IPO						$ 152.0
Secondary Public Offerings | Certain Officers and Former CEO | LSF8 Gypsum Holdings, L.P. Long-Term Incentive Plan, Earned Incentive Payment
Subsidiary, Sale of Stock [Line Items]
Incentive payments												$ 29.9
Secondary Public Offerings | LSF8 Gypsum Holdings, L.P.
Subsidiary, Sale of Stock [Line Items]
Shares issued at public offering (shares) | shares	5,106,803	4,600,000	361,747	4,600,000	5,000,000
Offering price per share (usd per share) | $ / shares	$ 16.10	$ 19.85	$ 21.90	$ 21.90	$ 19.40
Secondary Public Offerings | LSF8 Gypsum Holdings, L.P. | Repurchase from LSF8 in private transaction
Subsidiary, Sale of Stock [Line Items]
Shares repurchased (shares) | shares	900,000