Basis of Presentation and Accounting Policies - Cumulative Effect of applying ASC 606 to the Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	$ 379,755	$ 311,224	$ 281,343
Inventories	124,150	111,740	111,830
Prepaid expenses and other current assets	118,678	148,469	165,635
Total assets	4,620,840	3,558,639	3,546,014
Accrued expenses	504,240	452,547	443,961
Early access sales accrual	324,251	225,128	232,401
Other liabilities, current portion	60,357	36,456	34,373
Accumulated other comprehensive loss	307	(10,623)	(11,572)
Accumulated deficit	(4,540,005)	(5,111,443)	(5,119,723)
Cumulative effect adjustment for adoption of new accounting guidance			9,229	$ 0
Total liabilities and shareholders’ equity	4,620,840	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	34,575	29,881
Inventories	(95)	(90)
Prepaid expenses and other current assets	(25,148)	(17,166)
Total assets	9,332	12,625
Accrued expenses	(11,640)	8,586
Early access sales accrual	(17,885)	(7,273)
Other liabilities, current portion	21,407	2,083
Accumulated other comprehensive loss	121	949
Accumulated deficit	17,329	8,280
Total liabilities and shareholders’ equity	$ 9,332	$ 12,625