Significant Accounting Policies, Revenue Recognition - Adoption of Topic 606 (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Condensed Consolidated Balance Sheet [Abstract]
Current portion of deferred revenue	$ 160,589		$ 160,589		$ 125,336
Long-term portion of deferred revenue	556,586		556,586		108,026
Accumulated deficit	(1,282,809)		(1,282,809)		(1,241,034)
Noncontrolling interest in Akcea Therapeutics, Inc.	121,181		121,181		84,267
Total stockholders' equity	817,436		817,436		365,280
Condensed Consolidated Statement of Operations [Abstract]
Revenue	117,747	$ 112,273	262,165	$ 228,073
Income (loss) from operations	(50,281)	6,450	(53,583)	25,935
Net income (loss)	(56,573)	$ (3,085)	(67,381)	$ 5,878
Net income (loss) per share, basic and diluted (in dollars per share)		$ (0.02)		$ 0.05
Condensed Consolidated Statement of Cash Flows [Abstract]
Net income (loss)	(56,573)	$ (3,085)	(67,381)	$ 5,878
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Deferred contract revenue			483,814	55,588
Cash and cash equivalents at beginning of period			129,630	84,685	84,685
Cash and cash equivalents at end of period	805,490	132,991	805,490	132,991	129,630
Increase in revenue from amortization of milestone payments achieved under new revenue guidance	17,100			27,300
Decrease in revenue from amortization of payments using input method under new revenue guidance	(9,700)			(13,500)
Commercial Revenue [Member]
Condensed Consolidated Statement of Operations [Abstract]
Revenue	57,198	23,688	99,221	31,489
SPINRAZA Royalties [Member]
Condensed Consolidated Statement of Operations [Abstract]
Revenue	56,653	22,366	97,734	27,577
Licensing and Other Royalties [Member]
Condensed Consolidated Statement of Operations [Abstract]
Revenue	545	1,322	1,487	3,912
Research and Development Revenue Under Collaborative Agreements [Member]
Condensed Consolidated Statement of Operations [Abstract]
Revenue	$ 60,549	88,585	$ 162,944	196,584
Topic 606 Adjustment [Member]
Condensed Consolidated Balance Sheet [Abstract]
Current portion of deferred revenue					18,871
Long-term portion of deferred revenue					35,318
Topic 606 [Member]
Revenue Recognition [Abstract]
Cumulative effect of adoption on retained earnings					(53,600)
Topic 606 [Member] | As Previously Reported Under Topic 605 [Member]
Condensed Consolidated Balance Sheet [Abstract]
Current portion of deferred revenue					106,465
Long-term portion of deferred revenue					72,708
Accumulated deficit					(1,187,398)
Noncontrolling interest in Akcea Therapeutics, Inc.					87,847
Total stockholders' equity					418,719
Condensed Consolidated Statement of Operations [Abstract]
Revenue		104,152		214,456
Income (loss) from operations		(1,671)		12,318
Net income (loss)		$ (11,206)		$ (7,739)
Net income (loss) per share, basic and diluted (in dollars per share)		$ (0.09)		$ (0.06)
Condensed Consolidated Statement of Cash Flows [Abstract]
Net income (loss)		$ (11,206)		$ (7,739)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Deferred contract revenue				69,205
Cash and cash equivalents at beginning of period				84,685	84,685
Cash and cash equivalents at end of period		132,991		132,991
Topic 606 [Member] | As Previously Reported Under Topic 605 [Member] | Commercial Revenue [Member]
Condensed Consolidated Statement of Operations [Abstract]
Revenue		22,923		31,680
Topic 606 [Member] | As Previously Reported Under Topic 605 [Member] | SPINRAZA Royalties [Member]
Condensed Consolidated Statement of Operations [Abstract]
Revenue		22,366		27,577
Topic 606 [Member] | As Previously Reported Under Topic 605 [Member] | Licensing and Other Royalties [Member]
Condensed Consolidated Statement of Operations [Abstract]
Revenue		557		4,103
Topic 606 [Member] | As Previously Reported Under Topic 605 [Member] | Research and Development Revenue Under Collaborative Agreements [Member]
Condensed Consolidated Statement of Operations [Abstract]
Revenue		81,229		182,776
Topic 606 [Member] | Topic 606 Adjustment [Member]
Condensed Consolidated Balance Sheet [Abstract]
Current portion of deferred revenue					18,871
Long-term portion of deferred revenue					35,318
Accumulated deficit					(53,636)
Noncontrolling interest in Akcea Therapeutics, Inc.					(3,580)
Total stockholders' equity					(53,439)
Condensed Consolidated Statement of Operations [Abstract]
Revenue		8,121		13,617
Income (loss) from operations		8,121		13,617
Net income (loss)		$ 8,121		$ 13,617
Net income (loss) per share, basic and diluted (in dollars per share)		$ 0.07		$ 0.11
Condensed Consolidated Statement of Cash Flows [Abstract]
Net income (loss)		$ 8,121		$ 13,617
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Deferred contract revenue				(13,617)
Cash and cash equivalents at beginning of period				0	$ 0
Cash and cash equivalents at end of period		0		0
Topic 606 [Member] | Topic 606 Adjustment [Member] | Commercial Revenue [Member]
Condensed Consolidated Statement of Operations [Abstract]
Revenue		765		(191)
Topic 606 [Member] | Topic 606 Adjustment [Member] | SPINRAZA Royalties [Member]
Condensed Consolidated Statement of Operations [Abstract]
Revenue		0		0
Topic 606 [Member] | Topic 606 Adjustment [Member] | Licensing and Other Royalties [Member]
Condensed Consolidated Statement of Operations [Abstract]
Revenue		765		(191)
Topic 606 [Member] | Topic 606 Adjustment [Member] | Research and Development Revenue Under Collaborative Agreements [Member]
Condensed Consolidated Statement of Operations [Abstract]
Revenue		$ 7,356		$ 13,808