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:

(i) Recoveries from tenants represents amounts which tenants are contractually obligated to reimburse the Company for the tenants’ portion of actual Recoverable Costs incurred. Generally the Company’s leases provide for the tenants to reimburse the Company based on the tenants’ share of the actual costs incurred in proportion to the tenants’ share of leased space in the property.

(ii) Percentage rent represents amounts billable to tenants based on the tenants' actual sales volume in excess of levels specified in the lease contract.

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)	Three months ended September 30, 2019			Nine months ended September 30, 2019
Operating lease income
Fixed and in-substance fixed lease income	$	203,407			602,698
Variable lease income		59,872			185,317
Other lease related income, net
Above/below market rent amortization		11,086			31,334
Uncollectible amounts in lease income		(2,222	)		(3,667	)
Total lease income	$	272,143			815,682

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

(in thousands)	Future Minimum Rents as of
Year Ending December 31,	September 30, 2019			December 31, 2018
2019	$	198,877	⁽¹⁾		761,151
2020		765,960			693,848
2021		685,457			608,587
2022		593,471			516,369
2023		489,035			414,424
Thereafter		1,925,998			1,691,203
Total	$	4,658,798			4,685,582

⁽¹⁾ The future minimum rental income as of September 30, 2019 includes amounts due between October 1, 2019 and December 31, 2019.

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)	Three months ended September 30, 2019		Nine months ended September 30, 2019
Operating lease expense
Ground leases	$	3,514		10,714
Office leases		1,066		3,173
Total fixed operating lease expense		4,580		13,887
Variable lease expense
Ground leases		364		1,236
Office leases		137		410
Total variable lease expense		501		1,646
Total lease expense	$	5,081		15,533
Cash paid for amounts included in the measurement of operating lease liabilities
Operating cash flows for operating leases	$	3,684		11,096

Operating lease expense under the Company's ground and office leases was $5.1 million and $5.0 million during the three months ended September 30, 2019 and 2018, respectively, and $15.5 million and $14.2 million during the nine months ended September 30, 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 September 30, 2019, and provides a reconciliation to the Lease liability included in the accompanying Consolidated Balance Sheets:

(in thousands)	Lease liabilities
Year Ending December 31,	Ground Leases			Office Leases			Total
2019 ⁽¹⁾	$	2,668			1,281			3,949
2020		10,707			5,079			15,786
2021		10,674			3,969			14,643
2022		10,698			3,002			13,700
2023		10,914			2,301			13,215
Thereafter		564,080			6,344			570,424
Total undiscounted lease liabilities	$	609,741			21,976			631,717
Present value discount		(405,850	)		(2,286	)		(408,136	)
Lease liabilities	$	203,891			19,690			223,581
Weighted average discount rate	5.2%			3.9%
Weighted average remaining term	49.4 years			5.7 years

⁽¹⁾ The undiscounted lease liability maturities shown as of September 30, 2019, includes amounts due between October 1, 2019 and December 31, 2019.

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
Year Ending 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