Significant Accounting Policies, Revenue Recognition - Revenue Sources (Details) $ in Thousands	1 Months Ended				3 Months Ended				9 Months Ended
	Dec. 31, 2017 USD ($) PerformanceObligation	Feb. 28, 2017 USD ($) PerformanceObligation	May 31, 2015 USD ($) PerformanceObligation	Dec. 31, 2012 USD ($) PerformanceObligation	Sep. 30, 2018 USD ($)	Jun. 30, 2018 Agreement	Sep. 30, 2017 USD ($)	Jun. 30, 2015 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Aug. 31, 2018 PerformanceObligation	Dec. 31, 2016 PerformanceObligation
Revenue Recognition [Abstract]
Revenue					$ 145,395		$ 118,314		$ 407,559	$ 346,387
Period of time after billing when payment is received									3 months
Deferred Revenue [Abstract]
Revenue recognized					37,200		31,600		$ 80,400	84,400
Current portion of deferred revenue	$ 125,336				157,145				157,145
Long-term portion of deferred revenue	108,026				523,384				523,384
Total deferred revenue	233,362
Biogen [Member]
Revenue Recognition [Abstract]
Number of agreements | Agreement						2
Licensing and Other Royalty Revenue [Member]
Revenue Recognition [Abstract]
Revenue					12,746		1,727		14,232	5,639
SMA Collaboration with Biogen [Member]
Revenue Recognition [Abstract]
Upfront payment received	$ 25,000
Number of separate performance obligations | PerformanceObligation	1
Neurology [Member]
Revenue Recognition [Abstract]
Upfront payment received				$ 30,000
Milestone payment earned and amortized over period of performance							10,000
Number of separate performance obligations | PerformanceObligation				1
SPINRAZA [Member]
Revenue Recognition [Abstract]
Revenue					10,000
Number of separate performance obligations | PerformanceObligation												2
Bayer [Member]
Revenue Recognition [Abstract]
Number of separate performance obligations | PerformanceObligation		3
Deferred Revenue [Abstract]
Total deferred revenue	$ 9,300				6,000				6,000
Bayer [Member]
Revenue Recognition [Abstract]
Upfront payment received			$ 100,000					$ 100,000
Number of separate performance obligations | PerformanceObligation			3
Bayer [Member]
Revenue Recognition [Abstract]
Upfront payment received		$ 75,000
Number of separate performance obligations | PerformanceObligation		2
PTC Therapeutics [Member]
Revenue Recognition [Abstract]
Revenue					12,000		0		12,000	0
Number of separate performance obligations | PerformanceObligation											2
Deferred Revenue [Abstract]
Total deferred revenue	0				0				$ 0
PTC Therapeutics [Member] | TEGSEDI [Member]
Revenue Recognition [Abstract]
Revenue					$ 12,000
As Previously Reported Under Topic 605 [Member] | Topic 606 [Member]
Revenue Recognition [Abstract]
Revenue							120,911			335,367
Deferred Revenue [Abstract]
Current portion of deferred revenue	106,465
Long-term portion of deferred revenue	72,708
Total deferred revenue	179,173
As Previously Reported Under Topic 605 [Member] | Topic 606 [Member] | Licensing and Other Royalty Revenue [Member]
Revenue Recognition [Abstract]
Revenue							879			4,983
Topic 606 Adjustment [Member] | Topic 606 [Member]
Revenue Recognition [Abstract]
Revenue							(2,597)			11,020
Deferred Revenue [Abstract]
Current portion of deferred revenue	18,871
Long-term portion of deferred revenue	35,318
Total deferred revenue	54,189
Topic 606 Adjustment [Member] | Topic 606 [Member] | Biogen [Member]
Deferred Revenue [Abstract]
Total deferred revenue	24,200
Topic 606 Adjustment [Member] | Topic 606 [Member] | AstraZeneca [Member]
Deferred Revenue [Abstract]
Total deferred revenue	15,900
Topic 606 Adjustment [Member] | Topic 606 [Member] | Novartis [Member]
Deferred Revenue [Abstract]
Total deferred revenue	11,800
Topic 606 Adjustment [Member] | Topic 606 [Member] | Other Partners [Member]
Deferred Revenue [Abstract]
Total deferred revenue	$ 2,300
Topic 606 Adjustment [Member] | Topic 606 [Member] | Licensing and Other Royalty Revenue [Member]
Revenue Recognition [Abstract]
Revenue							$ 848			$ 656