Significant Accounting Policies, Revenue Recognition - Revenue Sources (Details)	1 Months Ended				3 Months Ended				6 Months Ended
	Dec. 31, 2017 USD ($) PerformanceObligation	Feb. 28, 2017 USD ($) PerformanceObligation	May 31, 2015 USD ($) PerformanceObligation	Dec. 31, 2012 USD ($) PerformanceObligation	Jun. 30, 2018 USD ($) Agreement	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 PerformanceObligation
Revenue Recognition [Abstract]
Period of time after billing when payment is received									3 months
Deferred Revenue [Abstract]
Revenue recognized					$ 33,300,000		$ 35,500,000		$ 62,000,000	$ 60,000,000
Current portion of deferred revenue	$ 125,336,000				160,589,000				160,589,000
Long-term portion of deferred revenue	108,026,000				$ 556,586,000				556,586,000
Total deferred revenue	233,362,000
Biogen [Member]
Revenue Recognition [Abstract]
Number of agreements | Agreement					2
SMA Collaboration with Biogen [Member]
Revenue Recognition [Abstract]
Upfront payment received	$ 25,000,000
Number of separate performance obligations | PerformanceObligation	1
Neurology [Member]
Revenue Recognition [Abstract]
Upfront payment received				$ 30,000,000
Milestone payment earned and amortized over period of performance						$ 10,000,000
Number of separate performance obligations | PerformanceObligation				1
SPINRAZA [Member]
Revenue Recognition [Abstract]
Milestone payment earned and recognized							$ 50,000,000
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	$ 9,300,000				$ 7,800,000				$ 7,800,000
Bayer [Member]
Revenue Recognition [Abstract]
Upfront payment received			$ 100,000,000					$ 100,000,000
Number of separate performance obligations | PerformanceObligation			3
Bayer [Member]
Revenue Recognition [Abstract]
Upfront payment received		$ 75,000,000
Number of separate performance obligations | PerformanceObligation		2
As Previously Reported Under Topic 605 [Member] | Topic 606 [Member]
Deferred Revenue [Abstract]
Current portion of deferred revenue	106,465,000
Long-term portion of deferred revenue	72,708,000
Total deferred revenue	179,173,000
Topic 606 Adjustment [Member]
Deferred Revenue [Abstract]
Current portion of deferred revenue	18,871,000
Long-term portion of deferred revenue	35,318,000
Total deferred revenue	54,189,000
Topic 606 Adjustment [Member] | Topic 606 [Member]
Deferred Revenue [Abstract]
Current portion of deferred revenue	18,871,000
Long-term portion of deferred revenue	35,318,000
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