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STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) |  |  |  |  |  | 3 Months Ended | 12 Months Ended | 48 Months Ended |  |  |  |  |  |  |  |  |  |  |  |  |  |
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Apr. 16, 2018 USD ($) $ / shares | Mar. 07, 2018 USD ($) $ / shares | Jan. 17, 2017 USD ($) shares | Jul. 14, 2016 USD ($) | Jan. 29, 2016 shares | Sep. 30, 2017 $ / shares shares | Sep. 30, 2019 USD ($) senior_executive $ / shares shares | Sep. 30, 2018 USD ($) $ / shares shares | Sep. 30, 2017 USD ($) $ / shares shares | Jan. 29, 2020 shares | Nov. 13, 2019 $ / shares | Jun. 19, 2018 shares | Jan. 31, 2018 shares | Oct. 02, 2017 | Aug. 31, 2016 USD ($) | May 18, 2016 | Mar. 22, 2016 USD ($) | Jul. 31, 2015 USD ($) | Mar. 31, 2015 USD ($) | Jun. 18, 2014 USD ($) | Feb. 27, 2014 | Aug. 31, 2011 USD ($) | Dec. 21, 2009 USD ($) | Dec. 31, 2008 shares
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options Outstanding & Exercisable Shares (in Shares) | shares |  |  |  |  |  | 350,000 |  | 350,000 | 350,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options Outstanding & Exercisable Weighted Average Exercise Price (in Dollars per Share) | $ / shares |  |  |  |  |  | $ 20.00 |  | $ 20.00 | $ 20.00 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, dividends, per share, cash paid (in Dollars per share) | $ / shares |  |  |  |  |  |  | $ 0.29 | $ 0.28 | $ 0.24 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based payment award, description |  |  |  |  |  |  | Options granted under the Incentive Plan may be either “incentive stock options” or nonqualified stock options, generally expire ten years after the date of grant and are granted at an exercise price of not less than 100% of the fair market value at the date of grant. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based payment award, expiration period |  |  |  |  | 10 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum percentage of exercise price at grant date fair value |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based payment award, number of shares authorized (in Shares) | shares |  |  |  |  |  |  | 3,984,510 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based payment award, options, vested in period, fair value | $ |  |  |  |  |  |  | $ 4,748,000 | $ 11,216,000 | $ 29,508,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Employee service share-based compensation, nonvested awards, compensation not yet recognized, share-based awards other than options | $ |  |  |  |  |  |  | $ 20,358,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Employee service share-based compensation, nonvested awards, compensation cost not yet recognized, period for recognition |  |  |  |  |  |  | 1 year 10 months 24 days |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchased during period, shares (in Shares) | shares |  |  |  |  |  |  | 37,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchased during period, value | $ |  |  |  |  |  |  | $ 372,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchased during period per share (in Dollars per share) | $ / shares |  |  |  |  |  |  | $ 9.92 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase program, authorized amount | $ |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 50,000,000 |  |  | $ 50,000,000 | $ 50,000,000 |  |  | $ 50,000,000 |  |
Stock repurchase program, remaining authorized repurchase amount | $ |  |  |  |  |  |  | $ 57,955,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares paid for tax withholding for share based compensation (in Shares) | shares |  |  |  |  |  |  | 85,847 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares paid for tax withholding for share based compensation, value | $ |  |  |  |  |  |  | $ 1,059,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares paid for tax withholding for share based compensation, value per share (in Dollars per share) | $ / shares |  |  |  |  |  |  | $ 12.34 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock issued during period, shares, ESOP | shares |  |  |  |  |  |  |  | 621,875 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock, Number of Shares Held by Shareholder | shares |  |  |  |  |  |  |  |  |  |  |  | 5,583,375 |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount | $ |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 35,092,000 |  |  |  |  |  |  |  |  |
Issuance of treasury stock in settlement of convertible debt | $ |  |  |  |  |  |  |  |  | $ 48,858,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
GS Direct [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Owned, Balance, Shares | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10,000,000
Incentive Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based payment award, number of shares authorized (in Shares) | shares |  |  |  |  | 3,350,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incentive Stock Options [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock issued during period, shares, new issues (in Shares) | shares |  |  |  |  | 600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based payment award, number of shares available for grant (in Shares) | shares |  |  |  |  |  |  | 270,937 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
2006 Equity Incentive Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share based payment award equity instruments other than options additional grants in future (in Shares) | shares |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vested (in Dollars per share) | $ / shares |  |  |  |  |  |  | $ 13.15 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Instruments grants in period (in shares) | shares |  |  |  |  |  |  | 1,262,270 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average grant date fair value (in Dollars per share) | $ / shares |  |  |  |  |  |  | $ 10.11 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares paid for tax withholding for share based compensation (in Shares) | shares |  |  |  |  |  |  | 3,861 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares paid for tax withholding for share based compensation, value | $ |  |  |  |  |  |  | $ 47,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares paid for tax withholding for share based compensation, value per share (in Dollars per share) | $ / shares |  |  |  |  |  |  | $ 12.16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Instruments vested in period (in shares) | shares |  |  |  |  |  |  | 361,152 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value | $ / shares |  |  |  |  |  |  | $ 12.96 | $ 14.89 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Stock [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based payment award, award vesting period |  |  |  |  |  |  | 3 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Stock [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based payment award, award vesting period |  |  |  |  |  |  | 4 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Performance Shares [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based payment award, award vesting period |  |  |  |  |  |  | 3 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Instruments vested in period, fair value | $ |  |  |  |  |  |  | $ 9,185,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Instruments grants in period (in shares) | shares |  |  |  |  |  | 0 | 1,262,270 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average grant date fair value (in Dollars per share) | $ / shares |  |  |  |  |  |  | $ 12.51 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Stock and Performance Shares [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Instruments grants in period (in shares) | shares |  |  |  |  |  |  | 734,270 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Executive Officer [Member] | Restricted Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based payment award, award vesting period |  |  |  |  |  |  | 4 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Post-vesting holding period |  |  |  |  |  |  | 2 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Instruments number of persons granted shares (in shares) | senior_executive |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Executive Officer [Member] | Performance Shares [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity instruments other than options, granted in period, fair value | $ |  |  |  |  |  |  | $ 3,576,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average grant date fair value (in Dollars per share) | $ / shares |  |  |  |  |  |  | $ 6.77 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Executive Officer [Member] | Restricted Stock and Performance Shares [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Instruments grants in period (in shares) | shares |  |  |  |  |  |  | 528,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Forecast [Member] | Executive Officer [Member] | Restricted Stock [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Instruments vested in period (in shares) | shares |  |  |  |  |  |  |  |  |  | 384,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Forecast [Member] | Executive Officer [Member] | Restricted Stock [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Instruments vested in period (in shares) | shares |  |  |  |  |  |  |  |  |  | 528,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends payable, amount per share (in Dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  |  | $ 0.0750 |  |  |  |  |  |  |  |  |  |  |  |  |
Convert. debt due 2017 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount | $ |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 100,000,000 |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.00% |
ESOP Loans [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock issued during period, value, ESOP | $ |  |  |  |  |  |  |  | $ 10,908,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
ESOP, weighted average purchase price of shares purchased (in Dollars per share) | $ / shares |  |  |  |  |  |  |  | $ 17.54 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incentive Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based payment award, number of shares authorized (in Shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  | 1,000,000 |  |  |  |  |  |  |  |  |  |  |
Revolver Due 2020 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility, Current Borrowing Capacity | $ |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 350,000 |  |  |  |  |  |  |
Senior Notes Two Thousand Twenty Two [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount | $ |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 600,000,000 |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.25% |  | 5.25% |  |  |  |  | 5.25% |  |  |
Original debt, amount | $ |  |  |  | $ 125,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Convertible Notes 2017 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Conversion, Converted Instrument, Amount | $ |  |  | $ 173,855,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt conversion, converted instrument, cash received | $ |  |  | 125,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of treasury stock in settlement of convertible debt | $ |  |  | $ 48,858,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Converted instrument, shares issued | shares |  |  | 1,954,993,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Special Dividends [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
STOCKHOLDERS' EQUITY AND EQUITY COMPENSATION (Details) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, dividends, per share, cash paid (in Dollars per share) | $ / shares | $ 1.00 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock, Dividends, Per Share, Declared | $ / shares |  | $ 1.00 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends, Common Stock, Cash | $ |  | $ 38,073,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments of Dividends | $ | $ 38,073,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

X
- Definition Common Stock, Number of Shares Held by Shareholder + References No definition available. + Details
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Namespace Prefix: | gff_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Debt Conversion, Converted Instrument, Cash Received + References No definition available. + Details
Name: | gff_DebtConversionConvertedInstrumentCashReceived
Namespace Prefix: | gff_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Maximum percentage of exercise price at grand date fair value. + References No definition available. + Details
Name: | gff_MaximumPercentageOfExercisePriceAtGrandDateFairValue
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