Fair Value Measurements

The tables below present information about our assets and liabilities that are regularly measured and carried at fair value and indicate the level within the fair value hierarchy of the valuation techniques we utilized to determine such fair value:



As of March 31, 2019 (In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	1,881.9	$	—	$	1,881.9	$	—
Marketable debt securities:
Corporate debt securities	1,835.7		—		1,835.7		—
Government securities	921.3		—		921.3		—
Mortgage and other asset backed securities	281.5		—		281.5		—
Marketable equity securities	904.0		19.5		884.5		—
Derivative contracts	104.4		—		104.4		—
Plan assets for deferred compensation	29.4		—		29.4		—
Total	$	5,958.2	$	19.5	$	5,938.7	$	—
Liabilities:
Derivative contracts	$	24.1	$	—	$	24.1	$	—
Contingent consideration obligations	421.3		—		—		421.3
Total	$	445.4	$	—	$	24.1	$	421.3



As of December 31, 2018 (In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	705.5	$	—	$	705.5	$	—
Marketable debt securities:
Corporate debt securities	2,459.2		—		2,459.2		—
Government securities	969.6		—		969.6		—
Mortgage and other asset backed securities	260.5		—		260.5		—
Marketable equity securities	615.4		51.7		563.7		—
Derivative contracts	66.9		—		66.9		—
Plan assets for deferred compensation	25.4		—		25.4		—
Total	$	5,102.5	$	51.7	$	5,050.8	$	—
Liabilities:
Derivative contracts	$	24.6	$	—	$	24.6	$	—
Contingent consideration obligations	409.8		—		—		409.8
Total	$	434.4	$	—	$	24.6	$	409.8

There have been no impairments of our assets measured and carried at fair value during the three months ended March 31, 2019. In addition, there were no changes in valuation techniques or inputs utilized or transfers between fair value measurement levels during the three months ended March 31, 2019. The fair value of Level 2 instruments classified as cash equivalents, marketable debt securities and our marketable equity security investment in Ionis Pharmaceuticals, Inc. (Ionis) were determined through third-party pricing services or an option pricing valuation model. For additional information on our agreement with Ionis, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2018 Form 10-K. For a description of our validation procedures related to prices provided by third-party pricing services and our option pricing valuation model, please read Note 1, Summary of Significant Accounting Policies - Fair Value Measurements, to our consolidated financial statements included in our 2018 Form 10-K.

Debt Instruments

The fair and carrying values of our debt instruments, which are Level 2 liabilities, are summarized as follows:



	As of March 31, 2019				As of December 31, 2018
(In millions)	Fair Value		Carrying Value		Fair Value		Carrying Value
2.900% Senior Notes due September 15, 2020	$	1,500.7	$	1,486.6	$	1,489.5	$	1,480.8
3.625% Senior Notes due September 15, 2022	1,022.0		995.8		1,000.4		995.5
4.050% Senior Notes due September 15, 2025	1,795.1		1,738.2		1,745.1		1,737.8
5.200% Senior Notes due September 15, 2045	1,838.0		1,722.6		1,802.6		1,722.4
Total	$	6,155.8	$	5,943.2	$	6,037.6	$	5,936.5

The fair values of each of our series of Senior Notes were determined through market, observable and corroborated sources. For additional information on our debt instruments, please read Note 12, Indebtedness, to our consolidated financial statements included in our 2018 Form 10-K.

Contingent Consideration Obligations

In connection with our acquisitions of Convergence Pharmaceuticals Ltd., Stromedix Inc. and Biogen International Neuroscience GmbH in 2015, 2012 and 2010, respectively, we agreed to make additional payments based upon the achievement of certain milestone events. The following table provides a roll forward of the fair values of our contingent consideration obligations, which includes Level 3 measurements:



	For the Three Months Ended March 31,
(In millions)	2019		2018
Fair value, beginning of period	$	409.8	$	523.6
Changes in fair value	11.5		(5.6		)
Payments	—		(20.0		)
Fair value, end of period	$	421.3	$	498.0

As of March 31, 2019 and December 31, 2018, $274.0 million and $265.0 million, respectively, of the fair value of our total contingent consideration obligations was reflected as a component of other long-term liabilities in our condensed consolidated balance sheets with the remaining balance reflected as a component of accrued expenses and other.

For the three months ended March 31, 2019, changes in the fair value of our contingent consideration obligations were primarily due to changes in the expected timing of the achievement of certain remaining development milestones, a decrease in interest rates used to revalue our contingent consideration liabilities and the passage of time.

For the three months ended March 31, 2018, changes in the fair value of our contingent consideration obligations were primarily due to an increase in the interest rate used to revalue our contingent consideration liabilities during the period.