All of the Company’s leases are classified as operating leases. The Company's Lease income is comprised of both fixed and variable income. 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 as either fixed or variable lease income based on the criteria specified in ASC Topic 842:

(in thousands)	December 31, 2020			December 31, 2019
Operating lease income
Fixed and in-substance fixed lease income	$	807,603			813,444
Variable lease income		247,384			247,861
Other lease related income, net:
Above/below market rent and tenant rent inducement amortization, net		42,219			45,392
Uncollectible straight line rent		(34,673	)		(7,002	)
Uncollectible amounts billable in lease income		(82,367	)		(5,394	)
Total lease income	$	980,166			1,094,301

During the year ended December 31, 2020, the Company experienced a higher rate of uncollectible lease income driven by changes in expectations of collectibility of both past due rents and recoveries and future rent steps given the impact of the pandemic on our tenants.

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, 2020
2021	$	754,396
2022		676,083
2023		578,023
2024		480,768
2025		372,377
Thereafter		1,329,274
Total	$	4,190,921

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, 2020		December 31, 2019
Fixed operating lease expense
Ground leases	$	13,716		13,982
Office leases		4,334		4,229
Total fixed operating lease expense		18,050		18,211
Variable lease expense
Ground leases		1,044		1,693
Office leases		585		552
Total variable lease expense		1,629		2,245
Total lease expense	$	19,679		20,456
Cash paid for amounts included in the measurement of operating lease liabilities
Operating cash flows for operating leases	$	15,003		14,815

Operating lease expense under the Company's ground and office leases was $19.7 million, $20.5 million and $19.1 million for the years ended December 31, 2020, 2019, and 2018 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, 2020, 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
2021	$	10,778			4,654			15,432
2022		10,837			3,379			14,216
2023		11,054			2,580			13,634
2024		11,103			2,114			13,217
2025		11,106			1,961			13,067
Thereafter		542,184			2,777			544,961
Total undiscounted lease liabilities	$	597,062			17,465			614,527
Present value discount		(392,848	)		(1,289	)		(394,137	)
Lease liabilities	$	204,214			16,176			220,390
Weighted average discount rate		5.2	%		3.8	%
Weighted average remaining term (in years)		48.1			5.0