The following tables summarize our financial instruments by significant categories and fair value measurement on a recurring basis as of December 31, 2023 and December 31, 2022 and the amortized cost, gross unrealized gains, gross unrealized losses, and fair value of our AFS investments:


(in millions)	Fair Value Level	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value		Cash and Cash Equivalents		Investments		Restricted Cash, Cash Equivalents and Investments
December 31, 2023
Cash equivalents:
Money market mutual funds	Level 1	$	183	$	—	$	—	$	183	$	96	$	—	$	87
U.S. treasuries	Level 2	7		—		—		7		5		—		2
Total cash equivalents		190		—		—		190		101		—		89
AFS Investments:
Asset-backed securities	Level 2	41		—		(1)		40		—		40		—
Corporate bonds	Level 2	135		1		—		136		—		103		33
Agency securities	Level 2	40		—		(1)		39		—		10		29
U.S. treasuries	Level 2	231		1		(1)		231		—		47		184
Certificate of deposit	Level 2	2		—		—		2		—		—		2
Other debt securities	Level 2	8		—		—		8		—		8		—
Total AFS Investments		$	457	$	2	$	(3)	$	456	$	—	$	208	$	248


(in millions)	Fair Value Level	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value		Cash and Cash Equivalents		Investments		Restricted Cash, Cash Equivalents and Investments
December 31, 2022
Cash equivalents:
Money market mutual funds	Level 1	$	314	$	—	$	—	$	314	$	225	$	—	$	89
U.S. treasuries	Level 2	18		—		—		18		18		—		—
Total cash equivalents		332		—		—		332		243		—		89
AFS Investments:
Asset-backed securities	Level 2	42		—		(2)		40		—		40		—
Corporate bonds	Level 2	140		—		(1)		139		—		112		27
Agency securities	Level 2	33		—		(1)		32		—		5		27
U.S. treasuries	Level 2	229		—		—		229		—		62		167
Certificate of deposit	Level 2	12		—		—		12		—		—		12
Other debt securities	Level 2	8		—		—		8		—		7		1
Total AFS Investments		$	464	$	—	$	(4)	$	460	$	—	$	226	$	234

We use an independent pricing source to determine the fair value of our securities. The independent pricing source utilizes various pricing models for each asset class, including the market approach. The inputs and assumptions for the pricing models are market observable inputs including trades of comparable securities, dealer quotes, credit spreads, yield curves and other market-related data.

We have not adjusted the prices obtained from the independent pricing service and we believe the prices received from the independent pricing service are representative of the prices that would be received to sell the assets at the measurement date (exit price).

We did not have any Level 3 financial instruments recognized in our balance sheets as of December 31, 2023 and December 31, 2022. There were no transfers between levels as of December 31, 2023 and December 31, 2022.


(in millions)	December 31, 2023
One year or less	$	93
Over one year through five years	332
Over five years through ten years	11
Over ten years	20
Total fair value	$	456

The gross proceeds from sales and maturities of AFS securities for the years ended December 31, 2023, 2022, and 2021 are presented below. We had immaterial gross realized gains and losses from sales of investments for the year ended December 31, 2021.


	Year Ended December 31,
(in millions)	2023		2022		2021

Gross realized losses	$	(1)	$	(18)	$	—
Gross proceeds from sales	150		227		162
Gross proceeds from maturities	137		253		187
Total	$	286	$	462	$	349

Unrealized losses on fixed income securities are principally caused by changes in market interest rates and the financial condition of the issuer. In analyzing an issuer's financial condition, we consider whether the securities are issued by the federal government or its agencies, whether downgrades by credit rating agencies have occurred, and industry analysts' reports. Actual maturities may differ from contractual maturities because borrowers may have the right to prepay obligations with or without prepayment penalties.

As of December 31, 2022, management recorded an impairment loss of $7 million on fixed income securities due to the probability that management could sell these securities before they recover in value. None of this impairment was credit-related. After this recognized impairment, gross unrealized losses related to AFS investments in an unrealized loss position were $4 million. Gross unrealized losses were immaterial at December 31, 2023 and December 31, 2021.

The fair value of our 2029 Notes and 2031 Notes was obtained from a third-party pricing service and is based on observable market inputs. As such, the fair value of the senior notes is considered Level 2 in the hierarchy for fair value measurement. As of December 31, 2023, our 2029 Notes and 2031 Notes were carried at their cost, net of issuance costs, and had a fair value of $443 million and $414 million, respectively. As of December 31, 2022, our 2029 Notes were carried at their cost, net of issuance costs, and had a fair value of $413 million.

Our 2021 Revolver is a floating rate debt. At December 31, 2023, the fair value of our 2021 Revolver approximated its carrying value (exclusive of issuance costs). The fair value of our floating rate debt is estimated based on a discounted cash flow, which incorporates credit spreads, market interest rates and contractual maturities to estimate the fair value and is considered Level 3 in the hierarchy for fair value measurement.