4. Available-for-Sale Securities and Fair Value Measurements

Available-for-Sale Securities

The estimated fair value of available-for-sale securities is based on quoted market prices for these or similar investments that were based on prices obtained from a commercial pricing service. Available-for-sale securities are summarized below:

	March 31, 2017
(In thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
U.S. government agencies	$	6,006	$	—	$	(1	)	$	6,005
U.S. commercial paper	62,878		—		—			62,878
Money market funds	99,942		—		—			99,942

Total	$	168,826	$	—	$	(1	)	$	168,825

	December 31, 2016
(In thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
U.S. government agencies	$	12,428	$	1	$	—	$	12,429
U.S. commercial paper	72,065		—		—		72,065
Money market funds	64,319		—		—		64,319

Total	$	148,812	$	1	$	—	$	148,813

As of March 31, 2017, all of the available-for-sale securities had contractual maturities within one year and the weighted average maturity of marketable securities was approximately one month.

Fair Value Measurements

Our available-for-sale securities are measured at fair value on a recurring basis and our debt is carried at the amortized cost basis. The estimated fair values were as follows:

	Estimated Fair Value Measurements as of March 31, 2017 Using:
Types of Instruments	Quoted Price in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
(In thousands)	Level 1		Level 2		Level 3		Total
Assets
U.S. government agencies	$	—	$	6,005	$	—	$	6,005
U.S. commercial paper	—		62,878		—		62,878
Money market funds	99,942		—		—		99,942

Total assets measured at estimated fair value	$	99,942	$	68,883	$	—	$	168,825

Liabilities
2023 Notes	$	—	$	227,248	$	—	$	227,248
Non-recourse notes due 2029 (the “2029 Notes”)	—		487,827		—		487,827

Total fair value of liabilities	$	—	$	715,075	$	—	$	715,075

	Estimated Fair Value Measurements as of December 31, 2016 Using:
Types of Instruments	Quoted Price in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
(In thousands)	Level 1		Level 2		Level 3		Total
Assets
U.S. government agencies	$	—	$	12,429	$	—	$	12,429
U.S. commercial paper	—		72,065		—		72,065
Money market funds	64,319		—		—		64,319

Total assets measured at estimated fair value	$	64,319	$	84,494	$	—	$	148,813

Liabilities
2023 Notes	$	—	$	202,125	$	—	$	202,125
2029 Notes	—		487,189		—		487,189

Total fair value of liabilities	$	—	$	689,314	$	—	$	689,314

The fair value of our marketable securities classified within Level 2 is based upon observable inputs that may include benchmark yields, reported trades, broker/dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers and reference data including market research publications.

The fair value of our 2023 Notes and of our 2029 Notes is based on recent trading prices of the instruments.