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Business Combinations - Pro Forma Information (Details) - Fermacell - USD ($) $ in Millions | 12 Months Ended
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Mar. 31, 2019 | Mar. 31, 2018
Business Acquisition [Line Items] |  |
Net sales | $ 2,506.6 | $ 2,367.0
Income before income taxes | 329.7 | 203.6
Europe Building Products |  |
Business Acquisition [Line Items] |  |
Net sales | 368.3 | 348.8
Income before income taxes | $ 39.1 | $ 29.0

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