During the years ended December 31, 2019, 2018, and 2017, we made improvements, excluding tenant-funded improvements, to our properties totaling $13.9 million, $56.3 million and $55.0 million, respectively.

We committed $25.5 million for expenditures related to 0.6 million square feet of leases executed during 2019, excluding leasing activity for assets during the quarter in which the asset was sold or classified as held for sale. Committed but unspent tenant related obligations are leasing commissions and tenant improvements. Based on existing leases as of December 31, 2019, committed but unspent tenant related obligations were $73.9 million.

During the year ended December 31, 2019, we sold the following properties, which did not represent strategic shifts under ASC Topic 2015 (dollars in thousands):


Property	Date Sold	Number of Properties	Number of Buildings	Square Footage	Gross Sales Price		Gain on Sale
1735 Market Street(1)	March 2019	1	1	1,286,936	$	451,600	$	192,985
600 108th Avenue NE(2)	April 2019	1	1	254,510	195,000		149,009
Research Park(3)	June 2019	1	4	1,110,007	165,500		78,158
		3	6	2,651,453	$	812,100	$	420,152

(1)Certain of our subsidiaries sold 100.0% of the equity interests in the fee simple owner of this property. The sale represents an individually significant disposition. The operating results of this property are included in continuing operations for all periods presented through the date of sale. Net income related to this property was $197.5 million (of which $193.0 million related to the gain on sale), $8.5 million and $5.6 million for the years ended December 31, 2019, 2018 and 2017 respectively.

(3)There is consideration of $2.0 million being held in escrow related to the sale of this property. To the extent any of these proceeds are ultimately released to the Company, the gain on sale will increase.

During the year ended December 31, 2018, we sold the following properties, which did not represent strategic shifts under ASC Topic 2015 (dollars in thousands):


Property	Date Sold	Number of Properties	Number of Buildings	Square Footage	Gross Sales Price		Gain (Loss) on Sale
1600 Market Street	February 2018	1	1	825,968	$	160,000	$	54,599
600 West Chicago Avenue(1)	February 2018	1	2	1,561,477	510,000		107,790
5073, 5075, & 5085 S. Syracuse Street	March 2018	1	1	248,493	115,186		42,762
1601 Dry Creek Drive	May 2018	1	1	552,865	68,500		26,979
777 East Eisenhower Parkway	August 2018	1	1	290,530	29,500		5,308
8750 Bryn Mawr Avenue	September 2018	1	2	636,078	141,000		15,194
97 Newberry Road	December 2018	1	1	289,386	7,100		(1,174)
		7	9	4,404,797	$	1,031,286	$	251,458

(1) The sale represents an individually significant disposition. The operating results of this property are included in continued operations for all periods presented through the date of sale. Net income for this property was $0.3 million, $110.6 million and $9.3 million for the years ended December 31, 2019, 2018 and 2017, respectively.

During the year ended December 31, 2017, we sold 16 properties (37 buildings) with a combined 6,588,128 square feet for an aggregate gross sales price of $862.6 million, excluding credits and closing costs.

Our real estate properties are generally leased on gross lease and modified gross lease bases pursuant to non-cancelable, fixed term operating leases expiring between 2020 and 2037. These gross leases and modified gross leases require us to pay all or some property operating expenses and to provide all or some property management services. A portion of these property operating expenses are reimbursed by the tenants.

The future minimum lease payments, excluding tenant reimbursement revenue, scheduled to be received by us during the current terms of our leases as of December 31, 2019 are as follows (in thousands):


	December 31,
	2019		2018		2017
Lease payments	$	81,698	$	129,539	$	250,564
Variable lease payments	35,171		54,829		73,851
Rental revenue	$	116,869	$	184,368	$	324,415