Leases

As lessor:

We lease space to tenants under operating leases. Most of the leases provide for the payment of fixed base rentals payable monthly in advance. Office building leases generally require the tenants to reimburse us for operating costs and real estate taxes above their base year costs. Certain leases provide for pass-through to tenants for the tenant’s share of real estate taxes, insurance and maintenance. Certain leases also provide for the payment by the lessee of additional rent based on a percentage of the tenants’ sales. As of December 31, 2018, future base rental revenue under non-cancelable operating leases, excluding rents for leases with an original term of less than one year and rents resulting from the exercise of renewal options, are as follows:



(Amounts in thousands)
Year Ending December 31:
2019	$	1,547,162
2020	1,510,097
2021	1,465,024
2022	1,407,615
2023	1,269,141
Thereafter	5,832,467

These amounts do not include percentage rentals based on tenants’ sales. These percentage rents approximated $4,746,000, $4,062,000 and $3,590,000, for the years ended December 31, 2018, 2017 and 2016, respectively.

None of our tenants accounted for more than 10% of total revenues in any of the years ended December 31, 2018, 2017 and 2016.

As lessee:

We are a tenant under operating leases for certain properties. These leases have terms that expire during the next ninety-nine years. Future minimum lease payments under operating leases at December 31, 2018 are as follows:



(Amounts in thousands)
Year Ending December 31:
2019	$	46,147
2020	45,258
2021	42,600
2022	43,840
2023	44,747
Thereafter	1,612,627

Rent expense, a component of “operating" expenses on our consolidated statements of income, was $41,063,000, $40,219,000 and $40,170,000 for the years ended December 31, 2018, 2017 and 2016, respectively.