Assets and liabilities subject to fair value measurements are as follows (in thousands):

	As of December 31, 2015
	Level 1		Level 2		Level 3		Balance
Assets
Money market funds(1)	$	496,434	$	—	$	—	$	496,434
Federally-sponsored and corporate debt securities(2)		—		159,830		—		159,830

Total assets	$	496,434	$	159,830	$	—	$	656,264


Liabilities
Convertible notes due 2016(3)	$	15,998	$	—	$	—	$	15,998
Contingent consideration(4)		—		—		9,400		9,400

Total liabilities	$	15,998	$	—	$	9,400	$	25,398

	As of December 31, 2014
	Level 1		Level 2		Level 3		Balance
Assets
Money market funds(1)	$	298,416	$	—	$	—	$	298,416
Federally-sponsored and corporate debt securities(2)		—		420,731		—		420,731

Total assets	$	298,416	$	420,731	$	—	$	719,147


Liabilities
Convertible notes due 2016(3)	$	388,153	$	—	$	—	$	388,153
Contingent consideration(4)		—		—		11,502		11,502

Total liabilities	$	388,153	$	—	$	11,502	$	399,655


(1)	Included in cash and cash equivalents on the accompanying consolidated balance sheets.
(2)	Included in current and non-current marketable investments on the accompanying consolidated balance sheets. The fair value of these securities is principally measured or corroborated by trade data for identical securities in which related trading activity is not sufficiently frequent to be considered a Level 1 input or comparable securities that are more actively traded. See also Note 4—Investments—Marketable Investments—Held-to-Maturity Investments to these consolidated financial statements.
(3)	Included in convertible notes on the accompanying consolidated balance sheets. The fair value of our Convertible Notes is estimated using Level 1 observable inputs since our Convertible Notes are trading with sufficient frequency such that we believe related pricing can be used as the primary basis for measuring their fair value. As of December 31, 2015 and December 31, 2014, the fair value of the Convertible Notes was substantially higher than their book value. This was primarily due to the excess conversion value of the notes compared to the notes' par value, and the fact that any such excess would be paid in shares of our common stock.
(4)	Included in other liabilities on the accompanying consolidated balance sheets. The fair value of contingent consideration has been estimated using probability weighted discounted cash flow models (DCF). The DCF incorporates Level 3 inputs including estimated discount rates that we believe market participants would consider relevant in pricing and the projected timing and amount of cash flows, which are estimated and developed, in part, based on the requirements specific to each acquisition agreement. We analyze and evaluate these fair value measurements quarterly to determine whether valuation inputs continue to be relevant and appropriate or whether current period developments warrant adjustments to valuation inputs and related measurements.