Basis of Presentation and Accounting Policies - Cumulative Effect of applying ASC 606 to the Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	$ 327,294	$ 311,224	$ 281,343
Inventories	117,346	111,740	111,830
Prepaid expenses and other current assets	109,886	148,469	165,635
Total assets	3,952,974	3,558,639	3,546,014
Accrued expenses	411,231	452,547	443,961
Early access sales accrual	268,446	225,128	232,401
Other liabilities, current portion	56,654	36,456	34,373
Accumulated other comprehensive loss	(39,743)	(10,623)	(11,572)
Accumulated deficit	(4,876,111)	(5,111,443)	(5,119,723)
Cumulative effect adjustment for adoption of new accounting guidance			9,229	$ 0
Total liabilities and shareholders’ equity	3,952,974	3,558,639	3,546,014
Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative effect adjustment for adoption of new accounting guidance			(24,120)
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	33,152	29,881
Inventories	(68)	(90)
Prepaid expenses and other current assets	(18,932)	(17,166)
Total assets	14,152	12,625
Accrued expenses	3,217	8,586
Early access sales accrual	(10,511)	(7,273)
Other liabilities, current portion	8,628	2,083
Accumulated other comprehensive loss	1,405	949
Accumulated deficit	11,413		$ 8,280
Total liabilities and shareholders’ equity	$ 14,152	$ 12,625