2018, due to improved collection levels, and increased as of June 30, 2018, compared to December 31, 2017, primarily due to higher business levels in June 2018 versus December 2017.
Cash, cash equivalents, and short-term investments increased $50.1 million from December 31, 2017 to June 30, 2018. During the six-month period ended June 30, 2018, cash provided by operations of $119.6 million was used to repay $33.7 million of notes payable; fund $22.7 million of capital expenditures (and an additional $14.4 million of certain Asset-Based revenue equipment was financed with notes payable), net of proceeds from asset sales; and pay dividends of $4.1 million on common stock.
Financing Arrangements
Our financing arrangements are discussed in Note F to our consolidated financial statements included in Part I, Item 1 of this Quarterly Report on Form 10-Q.
Contractual Obligations
We have purchase obligations, consisting of authorizations to purchase and binding agreements with vendors, relating to revenue equipment used in our Asset-Based operations, other equipment, facility improvements, software, certain service contracts, and other items for which amounts were not accrued in the consolidated balance sheet as of June 30, 2019. These purchase obligations totaled $122.2 million as of June 30, 2019, with $109.0 million estimated to be paid within the next year, $10.9 million estimated to be paid in the following two-year period, and $2.3 million to be paid within five years, provided that vendors complete their commitments to us. Purchase obligations for revenue equipment and other equipment are included in our 2019 capital expenditure plan. We also have contractual obligations for operating leases, primarily related to our Asset-Based service centers, as of June 30, 2019 which are disclosed in Note E to our consolidated financial statements included in Part I, Item 1 of this Quarterly Report on Form 10-Q.
Our contractual obligations related to our notes payable, which provide financing for revenue equipment and software purchases, totaled $184.6 million, including interest, as of June 30, 2019, a decrease of $10.4 million from December 31, 2018. The scheduled maturities of our long-term debt obligations as of June 30, 2019 are disclosed in Note F to our consolidated financial statements included in Part I, Item 1 of this Quarterly Report on Form 10-Q. There have been no other material changes in the contractual obligations disclosed in our 2018 Annual Report on Form 10-K during the six months ended June 30, 2019.
For 2019, our total capital expenditures, including amounts financed, are estimated to range from $170.0 million to $180.0 million, net of asset sales. These 2019 estimated net capital expenditures include revenue equipment purchases of $90.0 million, primarily for our Asset-Based operations. The remainder of 2019 expected capital expenditures includes real estate projects, costs of other facility and handling equipment for our Asset-Based operations, including forklifts, and technology investments across the enterprise. We have the flexibility to adjust certain planned 2019 capital expenditures as business levels dictate. Depreciation and amortization expense, excluding amortization of intangibles, is estimated to be in a range of $110.0 million to $115.0 million in 2019.
As previously disclosed within the Consolidated Results section of Results of Operations, our nonunion defined benefit pension plan was terminated with an effective date of December 31, 2017 and the liquidation of plan assets and settlement of plan obligations is expected to be completed in 2019. In August 2019, the plan received a preliminary bid for a nonparticipating annuity contract from an insurance company. Based on the most recently available actuarial information, including the preliminary bid, we estimate making a cash contribution to the plan to fund an annuity contract purchase and the remaining benefit distributions expected to be made from the plan in excess of plan assets of approximately $7.0 million, which would be deductible for income tax purposes, although there can be no assurances in this regard as the actual contribution amount is dependent on various factors and will be determined using updated actuarial data.
ABF Freight System, Inc. and certain other subsidiaries reported in our Asset-Based operating segment contribute to multiemployer health, welfare, and pension plans based generally on the time worked by their contractual employees, as specified in the collective bargaining agreement and other supporting supplemental agreements (see Note G to our consolidated financial statements included in Part I, Item 1 of this Quarterly Report on Form 10-Q).