Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2021

Fair Value Disclosures [Abstract]

Summary of assets and liabilities recorded at fair value

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 June 30, 2021
(In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	1,180.2	$	—	$	1,180.2	$	—
Marketable debt securities:
Corporate debt securities	1,382.5		—		1,382.5		—
Government securities	688.6		—		688.6		—
Mortgage and other asset backed securities	152.8		—		152.8		—
Marketable equity securities	1,593.5		273.3		1,320.2		—
Derivative contracts	20.8		—		20.8		—
Plan assets for deferred compensation	33.7		—		33.7		—
Total	$	5,052.1	$	273.3	$	4,778.8	$	—
Liabilities:
Derivative contracts	$	72.4	$	—	$	72.4	$	—
Contingent consideration obligations	226.3		—		—		226.3
Total	$	298.7	$	—	$	72.4	$	226.3

	As of December 31, 2020
(In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	626.9	$	—	$	626.9	$	—
Marketable debt securities:
Corporate debt securities	1,301.5		—		1,301.5		—
Government securities	627.1		—		627.1		—
Mortgage and other asset backed securities	122.4		—		122.4		—
Marketable equity securities	1,974.3		271.1		1,703.2		—
Derivative contracts	20.5		—		20.5		—
Plan assets for deferred compensation	28.2		—		28.2		—
Total	$	4,700.9	$	271.1	$	4,429.8	$	—
Liabilities:
Derivative contracts	$	217.2	$	—	$	217.2	$	—
Contingent consideration obligations	259.8		—		—		259.8
Total	$	477.0	$	—	$	217.2	$	259.8

Fair value measurements, nonrecurring

The gains or losses on assets measured at fair value on a nonrecurring basis, are summarized as follows:

	As of June 30, 2021
(In millions)	Beginning Book Value		Impairment		Ending Book Value
BIIB111 intangible asset	$	365.0	$	(350.0)	$	15.0
BIIB112 intangible asset	220.0		(191.6)		28.4

Summary of fair and carrying value of debt instruments

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

	As of June 30, 2021				As of December 31, 2020
(In millions)	Fair Value		Carrying Value		Fair Value		Carrying Value
3.625% Senior Notes due September 15, 2022	$	1,037.8	$	998.5	$	1,054.1	$	997.9
4.050% Senior Notes due September 15, 2025	1,945.3		1,742.0		2,003.1		1,741.2
2.250% Senior Notes due May 1, 2030	1,503.3		1,491.6		1,557.2		1,491.1
5.200% Senior Notes due September 15, 2045 (1)	1,483.1		1,099.7		2,365.1		1,723.4
3.150% Senior Notes due May 1, 2050	1,471.1		1,472.9		1,536.4		1,472.6
3.250% Senior Notes due February 15, 2051 (1)	700.1		464.5		—		—
Total	$	8,140.7	$	7,269.2	$	8,515.9	$	7,426.2

(1) In February 2021 we completed a private offer to exchange (Exchange Offer) our tendered 5.200% Senior Notes due September 15, 2045 (2045 Senior Notes), whereby approximately $624.6 million of our 2045 Senior Notes were exchanged for approximately $700.7 million of a new series of 3.250% Senior Notes due February 15, 2051 (2051 Senior Notes). For additional information on our Exchange Offer, please read Note 10, Indebtedness, to these condensed consolidated financial statements.

Fair value of contingent consideration obligations

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 June 30,				For the Six Months Ended June 30,
(In millions)	2021		2020		2021		2020
Fair value, beginning of period	$	226.0	$	341.6	$	259.8	$	346.1
Changes in fair value	0.3		10.0		(33.5)		5.5
Fair value, end of period	$	226.3	$	351.6	$	226.3	$	351.6