Leases

We lease substantially all our office space under operating leases and our equipment under operating and capital leases. Office leases may include renewal options. In circumstances where the exercise of a renewal option is reasonably assured at the inception of the lease, the renewal period is included in the determination of the lease term. Office leases may also include scheduled rent increases and concessions, such as rent abatements and landlord incentives and tenant improvement allowances. Scheduled rent increases are recognized on a straight-line basis over the lease term and concessions are recorded as deferred rent and are amortized in rent expense on a straight-line basis over the lease term. Certain office leases require payment of real estate taxes and other occupancy costs and these costs are not included in rent expense. Leasehold improvements made at inception or during the lease term are amortized over the shorter of the asset life or the lease term, which may include renewal periods where the renewal is reasonably assured.

Rent expense for the three years ended December 31, 2018 was (in millions):



	2018			2017			2016
Office base rent	$	295.0		$	336.7		$	339.7
Third-party sublease rent	(7.2		)	(6.3		)	(5.6		)
Net office rent	287.8			330.4			334.1
Equipment rent	16.6			20.1			21.0
	$	304.4		$	350.5		$	355.1

The aggregate minimum lease payments under non-cancelable operating leases, reduced by third-party sublease rent receivable from existing non-cancelable subleases, and capital leases are (in millions):



	Operating Leases			Capital Leases
2019	$	327.3		$	41.8
2020	278.2			38.2
2021	226.2			29.3
2022	186.1			15.6
2023	150.9			6.6
Thereafter	663.6			3.9
Total	1,832.3			135.4
Sublease rent	(14.1		)
Net operating lease payments	$	1,818.2
Interest component				(7.9		)
Present value of minimum capital lease payments				$	127.5

Assets under capital lease and capital lease obligations at December 31, 2018 and 2017 were (in millions):



	2018			2017
Assets under capital lease:
Cost	$	258.2		$	229.3
Accumulated depreciation	(133.8		)	(121.0		)
	$	124.4		$	108.3
Capital lease obligations:
Current	$	38.6		$	31.6
Long-term	88.9			78.9
	$	127.5		$	110.5

Depreciation expense for assets under capital lease was $36.7 million, $31.1 million and $27.2 million in 2018, 2017 and 2016, respectively.

As described in Note 22, on January 1, 2019, we will adopt FASB Accounting Standards Codification, or ASC, Topic 842, Leases, or ASC 842, which requires that right-of-use assets and related lease liabilities for all operating leases be recognized on the balance sheet.