Collaboration Agreements (Schedule of Collaboration Agreements) (Details) - USD ($) $ in Millions	1 Months Ended					12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2014	Jan. 31, 2014	Dec. 31, 2012	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Intangible Asset Balance						$ 10,858.1	$ 4,067.6
Acceleron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees						0.0	0.0	$ 0.0	$ 70.0
Milestones						0.0	0.0	17.0	27.5
Extension/ Termination of Agreements						0.0	0.0	0.0	0.0
Amortization of Prepaid Research and Development						0.0	0.0	0.0	0.0
Equity Investments Made During Period						0.0	52.4	10.0	30.5
Intangible Asset Balance						0.0	0.0
Equity Investment Balance						$ 224.9	$ 179.7
Percentage of Outstanding Equity						14.00%	14.00%
Acetylon
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees						$ 0.0	$ 0.0	50.0	0.0
Milestones						0.0	0.0	0.0	0.0
Extension/ Termination of Agreements						0.0	0.0	0.0	0.0
Amortization of Prepaid Research and Development						20.2	15.3	4.3	0.0
Equity Investments Made During Period						15.0	0.0	10.0	15.0
Intangible Asset Balance						0.2	20.4
Equity Investment Balance						$ 30.0	$ 25.0
Percentage of Outstanding Equity						14.00%	10.00%
Agios
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees						$ 9.0	$ 0.0	0.0	121.2
Milestones						0.0	0.0	0.0	0.0
Extension/ Termination of Agreements						0.0	20.0	20.0	20.0
Amortization of Prepaid Research and Development						0.0	0.0	0.0	0.0
Equity Investments Made During Period						0.0	38.3	12.8	37.5
Intangible Asset Balance						1.0	0.0
Equity Investment Balance						$ 340.4	$ 587.4
Percentage of Outstanding Equity						13.00%	14.00%
AstraZeneca
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees						$ 450.0
Milestones						0.0
Extension/ Termination of Agreements						0.0
Amortization of Prepaid Research and Development						0.0
Equity Investments Made During Period						0.0
Intangible Asset Balance						0.0
Bluebird
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees		$ 25.0				0.0	$ 0.0	74.7
Milestones						0.0	0.0	0.0
Extension/ Termination of Agreements						0.0	0.0	0.0
Amortization of Prepaid Research and Development						4.9	0.1	0.0
Equity Investments Made During Period						0.0	0.0	0.0
Intangible Asset Balance						20.2	0.1
Epizyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees						0.0	0.0	0.0	65.0
Milestones						0.0	0.0	25.0	0.0
Extension/ Termination of Agreements						10.0	0.0	0.0	0.0
Amortization of Prepaid Research and Development						0.0	0.0	0.0	0.0
Equity Investments Made During Period						0.0	9.9	1.0	25.0
Intangible Asset Balance						0.0	0.0
Equity Investment Balance						$ 58.9	$ 69.3
Percentage of Outstanding Equity						9.00%	11.00%
FORMA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees						$ 59.0	$ 225.0	52.8
Milestones						0.0	0.0	0.0
Extension/ Termination of Agreements						0.0	0.0	0.0
Amortization of Prepaid Research and Development						0.0	0.1	0.0
Equity Investments Made During Period						0.0	0.0	0.0
Intangible Asset Balance						0.1	0.1
Juno Therapeutics
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees		150.0				575.1
Milestones						0.0
Extension/ Termination of Agreements						0.0
Amortization of Prepaid Research and Development						0.0
Equity Investments Made During Period						424.9
Intangible Asset Balance						0.0
Equity Investment Balance						$ 401.8
Percentage of Outstanding Equity						9.00%
Lycera
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees		$ 82.5				$ 87.0
Milestones						0.0
Extension/ Termination of Agreements						0.0
Amortization of Prepaid Research and Development						0.0
Equity Investments Made During Period						10.0
Intangible Asset Balance						3.0
Equity Investment Balance						$ 10.0
Percentage of Outstanding Equity						8.00%
NantBioScience
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees				$ 50.0		$ 0.0	50.0
Milestones						0.0	0.0
Extension/ Termination of Agreements						0.0	0.0
Amortization of Prepaid Research and Development						0.0	0.0
Equity Investments Made During Period				$ 90.0		0.0	90.0
Intangible Asset Balance						0.0	0.0
Equity Investment Balance						$ 90.0	$ 90.0
Percentage of Outstanding Equity				14.00%		13.00%	14.00%
NantBioScience | Selling, general and administrative
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees							$ 25.0
Nurix
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees	$ 149.8					$ 149.8
Milestones						0.0
Extension/ Termination of Agreements						0.0
Amortization of Prepaid Research and Development						0.0
Equity Investments Made During Period						17.0
Intangible Asset Balance						0.2
Equity Investment Balance						$ 17.0
Percentage of Outstanding Equity						11.00%
OncoMed
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees						$ 2.5	2.5	155.0
Milestones						70.0	0.0	0.0
Extension/ Termination of Agreements						0.0	0.0	0.0
Amortization of Prepaid Research and Development						0.0	0.0	0.0
Equity Investments Made During Period						0.0	0.0	22.2
Intangible Asset Balance						0.0	0.0
Equity Investment Balance						$ 33.1	$ 32.0
Percentage of Outstanding Equity						5.00%	5.00%
Sutro
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees			$ 95.0		$ 35.0	$ 0.0	$ 72.6	0.0	26.3
Milestones						0.0	0.0	0.0	0.0
Extension/ Termination of Agreements						0.0	0.0	0.0	0.0
Amortization of Prepaid Research and Development						4.8	0.2	2.1	0.2
Equity Investments Made During Period						0.0	11.9	1.7	$ 4.0
Intangible Asset Balance						22.9	12.8
Equity Investment Balance						$ 17.6	$ 17.6
Percentage of Outstanding Equity						16.00%	15.00%
Other Collaboration Arrangements
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees						$ 69.8	$ 103.5	243.3
Milestones						8.0	8.3	1.0
Extension/ Termination of Agreements						8.1	0.0	0.0
Amortization of Prepaid Research and Development						0.9	7.5	0.9
Equity Investments Made During Period						50.0	55.7	$ 71.7
Intangible Asset Balance						25.0	34.4
Equity Investment Balance						$ 105.4	$ 132.7