Our total outstanding debt remained consistent at $3.1 billion during the first six months of 2021. Our total long-term debt to capitalization ratio (representing our long-term debt, including current maturities, divided by the sum of our long-term debt, redeemable noncontrolling interests, and our total stockholders’ equity) was 38.2% and 41.6% at June 30, 2021, and December 31, 2020, respectively.
Our unsecured credit agreement has a senior unsecured revolving credit facility (Facility), which provides a $1.2 billion unsecured Revolver, and matures in December 2024. Subject to certain conditions, we have the opportunity to increase the Facility size by $500.0 million. The unsecured Revolver is available to fund working capital, capital expenditures, and other general corporate purposes. The Facility contains financial covenants and other covenants pertaining to our ability to incur indebtedness and permit liens on property. Our ability to borrow funds within the terms of the unsecured Revolver is dependent upon our continued compliance with the financial and other covenants. At June 30, 2021, we had $1.2 billion of availability on the Revolver, $12.0 million of outstanding letters of credit and other obligations which reduce availability, and there were no borrowings outstanding.
The financial covenants under our Facility state that we must maintain an interest coverage ratio of not less than 2.50:1.00. Our interest coverage ratio is calculated by dividing our last-twelve-months (LTM) consolidated Adjusted EBITDA (earnings before interest, taxes, depreciation, amortization, and certain other non-cash transactions as allowed in the Facility) by our LTM gross interest expense, less amortization of financing fees. In addition, a debt to capitalization ratio of not more than 0.60:1.00 must be maintained. At June 30, 2021, our interest coverage ratio and debt to capitalization ratio were 22.38:1.00 and 0.38:1.00, respectively. We were, therefore, in compliance with these covenants at June 30, 2021, and we anticipate we will continue to be in compliance during the next twelve months.
Working Capital. We generated cash flow from operations of $849.4 million in the first six months of 2021 compared to $697.3 million in the comparable 2020 period. Operational working capital (representing amounts invested in trade receivables and inventories, less current liabilities other than income taxes payable and debt) increased $729.0 million, or 44%, to $2.4 billion at June 30, 2021, due primarily to increased accounts receivable and inventory, consistent with increased net sales and inventory costs.
Capital Investments. During the first six months of 2021, we invested $587.1 million in property, plant and equipment, primarily within our steel operations segment, compared with $527.3 million invested during the same period in 2020. We invested $488.5 million in our new Southwest-Sinton Flat Roll Steel Division in the first half of 2021, and $371.0 million in the first half of 2020. We entered 2021 with ample liquidity of $2.6 billion to provide for our planned 2021 capital requirements, including those necessary to finish construction of our new steel mill. For the second half of 2021, we estimate capital investments to be roughly $350 to $400 million, of which the new steel mill in Sinton, Texas, represents approximately $300 million.
Cash Dividends. As a reflection of continued confidence in our current and future cash flow generation ability and financial position, we increased our quarterly cash dividend by 4% to $0.26 per share in the first quarter 2021 (from $0.25 per share in 2020), resulting in declared cash dividends of $108.3 million during the first six months of 2021, compared to $105.2 million during the same period in 2020. We paid cash dividends of $107.6 million and $104.1 million during the first six months of 2021 and 2020, respectively. Our board of directors, along with executive management, approves the payment of dividends on a quarterly basis. The determination to pay cash dividends in the future is at the discretion of our board of directors, after taking into account various factors, including our financial condition, results of operations, outstanding indebtedness, current and anticipated cash needs and growth plans.
Other. In February 2020, our board of directors authorized share repurchase programs of up to $500.0 million of our common stock. Under the share repurchase programs, purchases take place as and when we determine in open market or private transactions made based upon the market price of our common stock, the nature of other investment opportunities or growth projects, our cash flows from operations, and general economic conditions. The share repurchase programs do not require us to acquire any specific number of shares, and may be modified, suspended, extended or terminated by us at any time. The share repurchase programs do not have an expiration date. There were $393.2 million