11. Commitments:

a. Operating leases

As of December 31, 2018, our estimated minimum rental commitments are as follows:


	(dollars in thousands)
2019	$	3,730
2020		1,413
2021		798
2022		608
2023		386
Thereafter		1,157

Our rental commitments are primarily for leases of railcars which are used to deliver coal to our coal-fired facilities. These railcar leasing costs are added to the cost of the fossil inventories and are recognized in fuel expense as the inventories are consumed. Rental expenses totaled $4,562,000, $4,919,000 and $4,456,000 in 2018, 2017 and 2016, respectively. We are assessing our future railcar needs and evaluating leasing options.

b. Fuel

To supply a portion of the fuel requirements to our generating units, Georgia Power, on our behalf for coal and Southern Nuclear on our behalf for nuclear fuel, have entered into various long-term commitments for the procurement of coal and nuclear fuel. The contracts in most cases contain provision for price escalations, minimum and maximum purchase levels and other financial commitments. The value of the coal commitments is based on maximum coal prices and minimum volumes as provided in the contracts and does not include taxes, transportation, government impositions or railcar costs.

We have entered into long-term agreements with various counterparties to provide firm natural gas transportation to our natural gas-fired facilities. The value of these agreements is based on fixed rates as provided in the contracts and does not include variable costs.

We have also entered into long-term maintenance agreements for certain of our natural gas-fired facilities. In most cases, these agreements include provisions for price escalation and performance bonuses and, if applicable, are included in the values; timing of expenditures is based on current operational assumptions. Certain agreements contain significant cancellation for convenience penalties and, therefore, amounts in the table below include total estimated expenditures over the life of the agreement. If these agreements were terminated by us in 2019 for convenience, our cancellation obligation would be approximately $80,000,000.

We have asset retirement obligations which are legal obligations to retire long-lived assets. These obligations are primarily for the decommissioning of our nuclear units and coal ash ponds. Expenditures are based on estimates determined through decommissioning studies and include provisions for price escalation and other factors. See Note 1h for information regarding our asset retirement obligations.

As of December 31, 2018, our estimated commitments are as follows:


	(dollars in thousands)
					Gas		Maintenance		Asset Retirement
	Coal		Nuclear Fuel		Transportation		Agreements		Obligations
2019	$	22,065	$	49,100	$	64,497	$	29,510	$	8,664
2020		7,091		27,500		63,260		53,911		11,274
2021		2,348		30,300		62,737		14,461		11,215
2022		—		25,400		53,796		14,929		11,200
2023		—		18,400		47,513		3,330		37,743
Thereafter		—		35,100		848,609		281,938		3,536,362