FAIR VALUE MEASUREMENTS (Tables)

12 Months Ended

Dec. 31, 2016

Fair Value of Financial Assets and Liabilities

The tables below present the fair value of these financial assets and liabilities determined using the following input levels.

	Fair Value Measurements at December 31, 2016
	Quoted Price in Active Markets For Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
Assets:
Cash and cash equivalents:
Overnight deposits	$	235,571	$	—	$	—	$	235,571
Money market instruments		—		172,759		—		172,759
Total cash and cash equivalents		235,571		172,759		—		408,330
Available-for-sale securities:
Short-term:
Certificates of deposit		—		2,800		—		2,800
Corporate debt securities		—		193,974		—		193,974
Commercial paper		—		16,075		—		16,075
U.S. government agency securities		—		168,499		—		168,499
Long-term:
Corporate debt securities		—		437,150		—		437,150
U.S. government agency securities		—		135,426		—		135,426
Greek government-issued bonds		—		134		—		134
Total available-for-sale securities		—		954,058		—		954,058
Other Current Assets:
Nonqualified Deferred Compensation Plan assets		—		163		—		163
Forward foreign currency exchange contract ⁽¹⁾		—		14,012		—		14,012
Restricted investments ⁽²⁾		—		3,754		—		3,754
Total other current assets		—		17,929		—		17,929
Other Assets:
Nonqualified Deferred Compensation Plan assets		—		9,121		—		9,121
Forward foreign currency exchange contract ⁽¹⁾		—		8,194		—		8,194
Strategic investment ⁽³⁾		4,064		—		—		4,064
Total other assets		4,064		17,315		—		21,379
Total assets	$	239,635	$	1,162,061	$	—	$	1,401,696
Liabilities:
Current Liabilities:
Nonqualified Deferred Compensation Plan liability	$	2,073	$	163	$	—	$	2,236
Forward foreign currency exchange contract⁽¹⁾		—		5,201		—		5,201
Contingent acquisition consideration payable		—		—		46,327		46,327
Total current liabilities		2,073		5,364		46,327		53,764
Other long-term liabilities:
Nonqualified Deferred Compensation Plan liability		17,303		9,121		—		26,424
Forward foreign currency exchange contract⁽¹⁾		—		2,342		—		2,342
Contingent acquisition consideration payable		—		—		115,310		115,310
Total other long-term liabilities		17,303		11,463		115,310		144,076
Total liabilities	$	19,376	$	16,827	$	161,637	$	197,840

	Fair Value Measurements at December 31, 2015
	Quoted Price in Active Markets For Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
Assets:
Cash and cash equivalents:
Overnight deposits	$	290,731	$	—	$	—	$	290,731
Money market instruments		—		106,309		—		106,309
Total cash and cash equivalents		290,731		106,309		—		397,040
Available-for-sale securities:
Short-term:
Certificates of deposit		—		56,951		—		56,951
Corporate debt securities		—		42,673		—		42,673
Commercial paper		—		12,733		—		12,733
U.S. government agency securities		—		83,222		—		83,222
Long-term:
Certificates of deposit		—		6,969		—		6,969
Corporate debt securities		—		315,240		—		315,240
U.S. government agency securities		—		103,316		—		103,316
Greek government-issued bonds		—		127		—		127
Total available-for-sale securities		—		621,231		—		621,231
Other Current Assets:
Nonqualified Deferred Compensation Plan assets		—		440		—		440
Forward foreign currency exchange contract ⁽¹⁾		—		10,478		—		10,478
Restricted investments ⁽²⁾		—		7,348		—		7,348
Total other current assets		—		18,266		—		18,266
Other Assets:
Nonqualified Deferred Compensation Plan assets		—		6,362		—		6,362
Forward foreign currency exchange contract ⁽¹⁾		—		3,533		—		3,533
Strategic investment ⁽³⁾		18,056		—		—		18,056
Total other assets		18,056		9,895		—		27,951
Total assets	$	308,787	$	755,701	$	—	$	1,064,488
Liabilities:
Current Liabilities:
Nonqualified Deferred Compensation Plan liability	$	1,151	$	440	$	—	$	1,591
Forward foreign currency exchange contract ⁽¹⁾		—		2,008		—		2,008
Contingent acquisition consideration payable		—		—		52,946		52,946
Total current liabilities		1,151		2,448		52,946		56,545
Other long-term liabilities:
Nonqualified Deferred Compensation Plan liability		24,341		6,362		—		30,703
Forward foreign currency exchange contract⁽¹⁾		—		3,057		—		3,057
Contingent acquisition consideration payable		—		—		32,663		32,663
Total other long-term liabilities		24,341		9,419		32,663		66,423
Total liabilities	$	25,492	$	11,867	$	85,609	$	122,968

(1)	See Note 11 to these Consolidated Financial Statements for further information regarding the derivative instruments.

(2)	The restricted investments at December 31, 2016 and 2015 secure the Company’s irrevocable standby letter of credit obtained in connection with certain commercial agreements.

(3)	The Company has investments in marketable equity securities measured using quoted prices in an active market that are considered strategic investments. See Note 6 to these Consolidated Financial Statements for additional discussion regarding the Company’s strategic investments.

Liabilities Measured at Fair Value Using Level 3 Inputs

Contingent acquisition consideration payable at December 31, 2015	$	85,609
Addition of contingent acquisition consideration payable related to the purchase of the Merck PKU Business		138,974
Changes in the fair value of contingent acquisition consideration payable for continuing development programs		6,825
Reduction of fair value related to termination of Kyndrisa development program		(43,652	)
Reduction of fair value related to termination of reveglucosidase alfa development program		(20,334	)
Foreign exchange remeasurement of Euro denominated contingent acquisition consideration payable		(5,785	)
Contingent acquisition consideration payable at December 31, 2016	$	161,637

Asset Retirement Obligation Liability and Corresponding Capital Asset

Asset retirement obligations at December 31, 2015	$	4,704
Accretion expense		107
Additions		—
Settlements and reversals		(665	)
Asset retirement obligations at December 31, 2016	$	4,146