The Company’s investments consist of available-for-sale debt investments, marketable equity investments, term deposit and certificate of deposit. The amortized cost, gross unrealized holding gains or losses, and fair value of the Company’s investments by major investments type are summarized in the tables below (in thousands):

	December 31, 2019
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Fair Value
US treasury notes and bills	$	426,995	$	536	$	(25	)	$	427,506
Certificates of deposit		30,000		32		—			30,032
Bond and mutual funds		10,730		86		—			10,816
Equity investments		125		130		—			255
Total investments	$	467,850	$	784	$	(25	)	$	468,609

	December 31, 2018
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Fair Value
US treasury notes and bills	$	467,296	$	109	$	(69	)	$	467,336
Term deposit		80,000		—		—			80,000
Certificates of deposit		30,000		—		(90	)		29,910
Bond and mutual funds		10,464		20		—			10,484
Equity investments		125		109		—			234
Total investments	$	587,885	$	238	$	(159	)	$	587,964

The contractual maturities of the available-for-sale investments and term deposit were as follows (in thousands):

The Company periodically reviews its available-for-sale investments and term deposit for other-than-temporary impairment. The Company considers factors such as the duration, severity and the reason for the decline in value, the potential recovery period and its intent to sell. For debt securities, the Company also considers whether (i) it is more likely than not that the Company will be required to sell the debt securities before recovery of their amortized cost basis, and (ii) the amortized cost basis cannot be recovered as a result of credit losses. During the three years ended December 31, 2019, the Company did not recognize any other-than-temporary impairment loss.

The Company started capitalizing inventory costs in June 2019 when FibroGen China began productions of roxadustat for commercial sales purposes. The provision to write-down excess and obsolete inventory was nominal for the year ended December 31, 2019.

Prepaid expenses and other current assets consisted of the following (in thousands):

	December 31,
	2019			2018
Unbilled contract assets	$	180,000		$	—
Deferred revenues from associated contracts		(54,790	)		—
Net unbilled contract assets		125,210			—
Prepaid assets		6,464			2,705
Other current assets		1,717			2,224
Total prepaid expenses and other current assets	$	133,391		$	4,929

The unbilled contract assets as of December 31, 2019 were related to two regulatory milestones totaling $130.0 million under the Europe Agreement with Astellas associated with the planned MAA submission in Europe, and a $50.0 million regulatory milestone under the U.S./RoW Agreement with AstraZeneca associated with the NDA submission in the U.S., which was submitted in December 2019 and accepted for review in February 2020. See Note 3 for details.

	December 31,
	2019			2018
Leasehold improvements	$	101,548		$	101,200
Building shell		—			53,880
Laboratory equipment		17,329			16,405
Machinery		8,217			8,382
Computer equipment		8,399			6,473
Furniture and fixtures		5,822			5,690
Construction in progress		1,792			367
Total property and equipment	$	143,107		$	192,397
Less: accumulated depreciation		(100,364	)		(65,199	)
Property and equipment, net	$	42,743		$	127,198

As of December 31, 2018, the Company had $53.9 million building shell cost and $13.5 million accumulated depreciation related to its building leases under build-to-suit arrangements. Upon the adoption of ASC 842 as of January 1, 2019, using the modified retrospective transition method, the Company derecognized these assets previously recognized under ASC 840 build-to-suit designation. Up to December 31, 2018, the leasehold improvements related to these building leases were depreciated over the life of the building under ASC 840. Upon the adoption of ASC 842, these leasehold improvements should have a useful life based on the lease term. As a result, at the adoption date, the Company recorded a cumulative adjustment of $38.9 million to the opening accumulated depreciation for these leasehold improvements so that their net balance equals the undepreciated amount had the useful life of the leasehold improvements always been equal to the lease terms. Refer to Note 2 for details.

Depreciation expense for the years ended December 31, 2019, 2018 and 2017 was $11.1 million, $6.6 million, and $6.1 million, respectively.

	December 31,
	2019		2018
Preclinical and clinical trial accruals	$	16,279	$	35,413
API product price adjustment		36,324		—
Payroll and related accruals		19,784		21,430
Property taxes and other		2,044		1,095
Professional services		4,842		2,648
Other		4,543		5,537
Total accrued liabilities	$	83,816	$	66,123

The amount of $36.3 million accrued as of December 31, 2019 was related to the change in estimated variable consideration of API product at the time the roxadustat listed price was issued by the Japanese Ministry of Health, Labour and Welfare. Refer to Note 3 for details.

	December 31,
	2019		2018
Accrued long-term co-promotional expenses	$	53,071	$	—
Other long-term tax liabilities		8,913		8,138
Operating lease liabilities, non-current		942		—
Other		1,340		1,855
Total other long-term liabilities	$	64,266	$	9,993

The accrued long-term co-promotional expenses of $53.1 million as of December 31, 2019 was related to the estimated amount payable to AstraZeneca for its sales and marketing efforts related to the commercial launch for roxadustat in China. The payment for such amount is not expected to occur within the next year.