Segment Information - Net Product Sales (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue from External Customer [Line Items]
Long-Lived Assets	$ 1,163.3				$ 1,471.5				$ 1,163.3	$ 1,471.5
Total revenues	1,384.3	$ 1,263.1	$ 1,203.3	$ 1,140.4	1,128.8	$ 1,026.5	$ 1,045.0	$ 930.9	$ 4,991.1	$ 4,131.2	$ 3,551.1
Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Revenue from External Customer [Line Items]
Percentage change in product sales									20.80%	16.40%
Soliris
Revenue from External Customer [Line Items]
Total revenues									$ 3,946.4	$ 3,563.0	3,144.1
Soliris | Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Revenue from External Customer [Line Items]
Percentage change in product sales									10.80%	13.30%
Ultomiris [Member]
Revenue from External Customer [Line Items]
Total revenues									$ 338.9	$ 0.0	0.0
Strensiq
Revenue from External Customer [Line Items]
Total revenues									$ 592.5	$ 475.1	339.8
Strensiq | Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Revenue from External Customer [Line Items]
Percentage change in product sales									24.70%	39.80%
Kanuma
Revenue from External Customer [Line Items]
Total revenues									$ 112.2	$ 92.0	65.6
Kanuma | Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Revenue from External Customer [Line Items]
Percentage change in product sales									22.00%	40.20%
Product [Member]
Revenue from External Customer [Line Items]
Total revenues									$ 4,990.0	$ 4,130.1	3,549.5
United States [Member]
Revenue from External Customer [Line Items]
Long-Lived Assets	272.8				468.3				272.8	468.3
United States [Member] | Soliris
Revenue from External Customer [Line Items]
Total revenues									$ 2,014.0	$ 1,588.4	1,235.0
United States [Member] | Soliris | Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Revenue from External Customer [Line Items]
Percentage change in product sales									26.80%	28.60%
United States [Member] | Ultomiris [Member]
Revenue from External Customer [Line Items]
Total revenues									$ 236.8	$ 0.0	0.0
United States [Member] | Strensiq
Revenue from External Customer [Line Items]
Total revenues									$ 451.7	$ 374.3	280.1
United States [Member] | Strensiq | Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Revenue from External Customer [Line Items]
Percentage change in product sales									20.70%	33.60%
United States [Member] | Kanuma
Revenue from External Customer [Line Items]
Total revenues									$ 60.0	$ 51.3	42.4
United States [Member] | Kanuma | Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Revenue from External Customer [Line Items]
Percentage change in product sales									17.00%	21.00%
Europe [Member]
Revenue from External Customer [Line Items]
Long-Lived Assets	889.6				1,001.1				$ 889.6	$ 1,001.1
Europe [Member] | Soliris
Revenue from External Customer [Line Items]
Total revenues									$ 1,049.8	$ 1,036.7	985.2
Europe [Member] | Soliris | Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Revenue from External Customer [Line Items]
Percentage change in product sales									1.30%	5.20%
Europe [Member] | Ultomiris [Member]
Revenue from External Customer [Line Items]
Total revenues									$ 52.2	$ 0.0	0.0
Europe [Member] | Strensiq
Revenue from External Customer [Line Items]
Total revenues									$ 77.0	$ 61.7	35.6
Europe [Member] | Strensiq | Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Revenue from External Customer [Line Items]
Percentage change in product sales									24.80%	73.30%
Europe [Member] | Kanuma
Revenue from External Customer [Line Items]
Total revenues									$ 27.1	$ 21.6	14.6
Europe [Member] | Kanuma | Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Revenue from External Customer [Line Items]
Percentage change in product sales									25.50%	47.90%
Asia Pacific [Member] | Soliris
Revenue from External Customer [Line Items]
Total revenues									$ 423.5	$ 382.0	328.1
Asia Pacific [Member] | Soliris | Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Revenue from External Customer [Line Items]
Percentage change in product sales									10.90%	16.40%
Asia Pacific [Member] | Ultomiris [Member]
Revenue from External Customer [Line Items]
Total revenues									$ 49.9	$ 0.0	0.0
Asia Pacific [Member] | Strensiq
Revenue from External Customer [Line Items]
Total revenues									$ 50.4	$ 27.9	18.6
Asia Pacific [Member] | Strensiq | Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Revenue from External Customer [Line Items]
Percentage change in product sales									80.60%	50.00%
Asia Pacific [Member] | Kanuma
Revenue from External Customer [Line Items]
Total revenues									$ 4.6	$ 3.7	2.7
Asia Pacific [Member] | Kanuma | Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Revenue from External Customer [Line Items]
Percentage change in product sales									24.30%	37.00%
Non-US, Excluding Europe And Asia Pacific [Member] | Soliris
Revenue from External Customer [Line Items]
Total revenues									$ 459.1	$ 555.9	595.8
Non-US, Excluding Europe And Asia Pacific [Member] | Soliris | Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Revenue from External Customer [Line Items]
Percentage change in product sales									(17.40%)	(6.70%)
Non-US, Excluding Europe And Asia Pacific [Member] | Ultomiris [Member]
Revenue from External Customer [Line Items]
Total revenues									$ 0.0	$ 0.0	0.0
Non-US, Excluding Europe And Asia Pacific [Member] | Strensiq
Revenue from External Customer [Line Items]
Total revenues									$ 13.4	11.2	5.5
Non-US, Excluding Europe And Asia Pacific [Member] | Strensiq | Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Revenue from External Customer [Line Items]
Percentage change in product sales									19.60%
Non-US, Excluding Europe And Asia Pacific [Member] | Kanuma
Revenue from External Customer [Line Items]
Total revenues									$ 20.5	15.4	$ 5.9
Non-US, Excluding Europe And Asia Pacific [Member] | Kanuma | Sales Revenue, Net [Member] | Geographic Concentration Risk [Member]
Revenue from External Customer [Line Items]
Percentage change in product sales									33.10%
Other Country [Member]
Revenue from External Customer [Line Items]
Long-Lived Assets	$ 0.9				$ 2.1				$ 0.9	$ 2.1