Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Cash flows from operating activities:
Net income (loss)	$ (399,106)	$ 22,708
Adjustments for non-cash operating items:
Change in fair value of contingent consideration	690,287	5,013
Stock-based compensation	28,905	27,450
Depreciation on property and equipment	5,115	3,085
Amortization of debt discount and debt issuance costs	170	7,827
Change in deferred income taxes	(229,169)	(773)
Excess tax benefits from stock-based compensation	(13,444)	(13,572)
Loss on extinguishment of Convertible Notes		7,871
Amortization of deferred revenue		(2,257)
Other non-cash items	(4,111)	(60)
Changes in operating assets and liabilities:
Receivable from collaboration partner	164,248	(12,420)
Prepaid expenses and other current assets	(19,964)	(6,085)
Other non-current assets	(394)	2,689
Accounts payable, accrued expenses and other current liabilities	(14,273)	27,153
Current taxes payable	(32,565)	(2,106)
Other non-current liabilities	2,846	718
Net cash provided by operating activities	178,545	67,241
Cash flows from investing activities:
Purchases of available-for-sale securities	(80,967)	(76,397)
Purchases of property and equipment	(13,932)	(6,478)
Change in restricted cash		(1,574)
Net cash used in investing activities	(94,899)	(84,449)
Cash flows from financing activities:
Principal repayment of borrowings under Revolving Credit Facility	(75,000)
Proceeds from issuance of common stock under equity incentive and stock purchase plans	19,853	14,699
Excess tax benefits from stock-based compensation	13,444	13,572
Reduction of build-to-suit lease obligation	(35)	(470)
Principal repayment of Convertible Notes		(90,994)
Cash settlement of Convertible Notes conversion premium		(1,126)
Net cash used in financing activities	(41,738)	(64,319)
Net change in cash and cash equivalents	41,908	(81,527)
Cash and cash equivalents at beginning of period	225,853	502,677
Cash and cash equivalents at end of period	267,761	421,150
Non-cash investing and financing activities:
Property and equipment expenditures incurred but not yet paid	$ 361	1,650
Reacquisition of Convertible Note equity component upon conversion		135,988
Fair value of common stock issued for conversion of Convertible Notes		131,084
Derecognition of build-to-suit lease asset		3,241
Derecognition of build-to-suit lease liability		3,176
Interest capitalized during construction period for build-to-suit lease transactions		1,070
Accrued interest payable forfeited upon conversion of Convertible Notes		292
Amounts capitalized under build-to-suit lease transactions		$ 283