Leases

We lease 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, 2017 was (in millions):



	2017			2016			2015
Office base rent	$	336.7		$	339.7		$	342.5
Third party sublease rent	(6.3		)	(5.6		)	(11.0		)
Net office rent	330.4			334.1			331.5
Equipment rent	20.1			21.0			22.6
	$	350.5		$	355.1		$	354.1

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
2018	$	317.8		$	34.5
2019	249.7			30.0
2020	198.3			26.4
2021	169.4			17.1
2022	144.2			6.4
Thereafter	627.2			3.1
Total	1,706.6			117.5
Sublease rent	(19.0		)
Net operating lease payments	$	1,687.6
Interest component				(7.0		)
Present value of minimum capital lease payments				$	110.5

Property under capital lease and capital lease obligations at December 31, 2017 and 2016 were (in millions):



	2017			2016
Property under capital lease:
Cost	$	229.3		$	184.8
Accumulated depreciation	(121.0		)	(110.2		)
	$	108.3		$	74.6
Capital lease obligations:
Current	$	31.6		$	24.2
Long-term	78.9			49.8
	$	110.5		$	74.0

Depreciation expense for property under capital lease was $31.1 million, $27.2 million and $26.5 million in 2017, 2016 and 2015, respectively.