The Company's Lease income is comprised of both fixed and variable income, as follows:

Fixed and in-substance fixed lease income includes stated amounts per the lease contract, which are primarily related to base rent, and in some cases stated amounts for CAM, real estate taxes, and insurance. Income for these amounts is recognized on a straight- line basis.

Variable lease income includes the following two main items in the lease contracts:

The following table provides a disaggregation of lease income recognized under ASC Topic 842, Leases, as either fixed or variable lease income based on the criteria specified in ASC 842:

(in thousands)	December 31, 2019
Operating lease income
Fixed and in-substance fixed lease income	$	806,442
Variable lease income		247,861
Other lease related income, net:
Above/below market rent and tenant rent inducement amortization		45,392
Uncollectible amounts in lease income		(5,394	)
Total lease income	$	1,094,301

Future minimum rents under non-cancelable operating leases, excluding variable lease payments, are as follows:

(in thousands)
For the year ended December 31,	December 31, 2019
2020	$	775,723
2021		706,016
2022		615,224
2023		511,104
2024		411,308
Thereafter		1,500,745
Total	$	4,520,120

(in thousands)
For the year ended December 31,	December 31, 2018
2019	$	761,151
2020		693,848
2021		608,587
2022		516,369
2023		414,424
Thereafter		1,691,203
Total	$	4,685,582

The Company has shopping centers that are subject to non-cancelable, long-term ground leases where a third party owns the underlying land and has leased the land to the Company to construct and/or operate a shopping center.

The Company has 22 properties within its consolidated real estate portfolio that are either partially or completely on land subject to ground leases with third parties. Accordingly, the Company owns only a long-term leasehold or similar interest in these properties. These ground leases expire through the year 2101, and in most cases, provide for renewal options.

In addition, the Company has non-cancelable operating leases pertaining to office space from which it conducts its business. Office leases expire through the year 2029, and in many cases, provide for renewal options.

The ground and office lease expense is recognized on a straight-line basis over the term of the leases, including management's estimate of expected option renewal periods. Operating lease expense under the Company's ground and office leases was as follows, including straight-line rent expense and variable lease expenses such as CPI increases, percentage rent and reimbursements of landlord costs:

(in thousands)	December 31, 2019
Fixed operating lease expense
Ground leases	$	13,982
Office leases		4,229
Total fixed operating lease expense		18,211
Vaiable lease expense
Ground leases		1,693
Office leases		552
Total variable lease expense		2,245
Total lease expense	$	20,456
Cash paid for amounts included in the measurement of operating lease liabilities
Operating cash flows for operating leases	$	14,815

Operating lease expense under the Company's ground and office leases was $20.5 million, $19.1 million and $18.4 million for the years ended December 31, 2019, 2018, and 2017 respectively, which includes fixed and variable rent expense.

The following table summarizes the undiscounted future cash flows by year attributable to the operating lease liabilities under ground and office leases as of December 31, 2019, and provides a reconciliation to the Lease liability included in the accompanying Consolidated Balance Sheets:

(in thousands)	Lease Liabilities
For the year ended December 31,	Ground Leases			Office Leases			Total
2020	$	10,697			5,152			15,849
2021		10,671			4,149			14,820
2022		10,698			3,188			13,886
2023		10,915			2,410			13,325
2024		10,964			1,939			12,903
Thereafter		553,116			4,404			557,520
Total undiscounted lease liabilities	$	607,061			21,242			628,303
Present value discount		(403,237	)		(2,148	)		(405,385	)
Lease liabilities	$	203,824			19,094			222,918
Weighted average discount rate		5.2	%		3.9	%
Weighted average remaining term (in years)		49.2			5.5

The following table summarizes the future obligations under non-cancelable operating leases, excluding unexercised renewal options, as of December 31, 2018:

(in thousands)	Future Lease Obligations
For the year ended December 31,	Ground Leases		Office Leases		Total
2019	$	10,672		4,405		15,077
2020		10,439		4,294		14,733
2021		10,344		3,549		13,893
2022		10,258		2,893		13,151
2023		10,369		2,189		12,558
Thereafter		461,762		5,944		467,706
Total	$	513,844		23,274		537,118