17 property, plant and equipment


				Buildings and
		Network		leasehold						Assets under
(millions)	Note	assets 1		improvements		Other 1		Land		construction		Total
At cost
As at January 1, 2017		$	28,284	$	2,954	$	1,021	$	55	$	592	$	32,906
Additions 2			972		51		44		—		1,426		2,493
Additions arising from business acquisitions			25		8		9		—		—		42
Dispositions, retirements and other			(1,724)		(63)		(48)		(7)		—		(1,842)
Assets under construction put into service			1,167		127		69		—		(1,363)		—
As at December 31, 2017			28,724		3,077		1,095		48		655		33,599
Additions 2			1,039		27		37		—		1,265		2,368
Additions arising from business acquisitions	18(b)		4		13		9		—		—		26
Dispositions, retirements and other			(767)		56		(52)		—		—		(763)
Assets under construction put into service			956		100		85		—		(1,141)		—
As at December 31, 2018		$	29,956	$	3,273	$	1,174	$	48	$	779	$	35,230
Accumulated depreciation
As at January 1, 2017		$	19,950	$	1,836	$	656	$	—	$	—	$	22,442
Depreciation			1,396		106		115		—		—		1,617
Dispositions, retirements and other			(1,708)		(58)		(62)		—		—		(1,828)
As at December 31, 2017			19,638		1,884		709		—		—		22,231
Depreciation			1,431		115		123		—		—		1,669
Dispositions, retirements and other			(769)		51		(43)		—		—		(761)
As at December 31, 2018		$	20,300	$	2,050	$	789	$	—	$	—	$	23,139
Net book value
As at December 31, 2017		$	9,086	$	1,193	$	386	$	48	$	655	$	11,368
As at December 31, 2018		$	9,656	$	1,223	$	385	$	48	$	779	$	12,091

As at December 31, 2018, the net carrying amount of assets under finance leases included in network assets was $100 (2017 – $NIL) and included in other was $1 (2017 – $NIL).

For the year ended December 31, 2018, additions include $(15) (2017 – $7) in respect of asset retirement obligations (see Note 25).

As at December 31, 2018, our contractual commitments for the acquisition of property, plant and equipment totalled $177 million over a period ending December 31, 2022 (2017 – $184 million over a period ending December 31, 2019).