Leases

We lease land, office and warehouse space, and certain office equipment under operating leases that expire at various dates through November 30, 2060.

Rent expense for our operating leases was $4.2 million, $4.0 million and $3.6 million for the years ended December 31, 2017, 2016 and 2015, respectively.

Estimated future minimum non-cancelable operating lease payments at December 31, 2017 were as follows:



(In thousands)	Amount
2018	$	4,411
2019	3,333
2020	2,347
2021	2,204
2022	1,649
Thereafter	443
Total operating lease commitments	$	14,387

Non-level Rents and Lease Incentives

Some of our leases are subject to rent escalations. For these leases, we recognize rent expense for the total contractual obligation utilizing the straight-line method over the lease term, ranging from 48 months to 125 months. The related short-term liability is recorded in other accrued liabilities (see Note 11) and the related long term liability is recorded in other long-term liabilities. The total liability related to rent escalations was $1.2 million and $1.1 million at December 31, 2017 and 2016, respectively.

The lease agreement for our corporate headquarters contains an allowance for moving expenses and tenant improvements of $1.5 million. These moving and tenant improvement allowances are recorded within other accrued liabilities and other long-term liabilities, depending on the short-term or long-term nature, and are being amortized as a reduction of rent expense over the 125-month term of the lease, which began on May 15, 2012.

Rental Costs During Construction

Rental costs associated with operating leases incurred during a construction period are expensed.

Prepaid Land Leases

We operate one factory within the PRC on which the land is leased from the government as of December 31, 2017. This land lease was prepaid to the PRC government at the time our subsidiary occupied the land. We have obtained land-use right certificate for the land pertaining to this factories.

The factory is located in the city of Yangzhou in the Jiangsu province. The remaining net book value of this prepaid lease was $2.6 million on December 31, 2017, and will be amortized on a straight-line basis over the remaining term of approximately 41 years. The buildings located on this land had a net book value of $20.9 million on December 31, 2017 and will be depreciated over a remaining weighted average period of 22 years.

The remaining net book value of this prepaid land lease is included within prepaid expenses and other current assets and other assets, depending on the short-term or long-term nature.