Revenue Recognition (Disaggregation of Revenue by Product and Geographical Region) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	$ 1,444.6	$ 1,203.3	$ 2,889.4	$ 2,343.7
SOLIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	975.5	980.8	1,998.4	1,942.8
ULTOMIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	251.1	54.2	473.9	78.8
STRENSIQ
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	184.3	141.3	356.5	271.4
KANUMA
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	33.6	26.2	60.3	49.7
United States | SOLIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	553.3	496.3	1,109.5	960.0
United States | ULTOMIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	158.1	54.2	289.6	78.8
United States | STRENSIQ
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	140.7	106.2	268.8	205.7
United States | KANUMA
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	15.4	15.3	31.8	29.1
Europe | SOLIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	247.9	280.2	511.4	544.7
Europe | ULTOMIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	32.0	0.0	65.8	0.0
Europe | STRENSIQ
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	18.3	19.5	42.3	37.0
Europe | KANUMA
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	8.4	6.8	15.9	13.1
Asia Pacific | SOLIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	82.4	110.3	169.5	211.2
Asia Pacific | ULTOMIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	59.6	0.0	116.7	0.0
Asia Pacific | STRENSIQ
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	15.0	12.1	28.6	22.0
Asia Pacific | KANUMA
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	0.9	1.3	1.8	2.1
Rest of World | SOLIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	91.9	94.0	208.0	226.9
Rest of World | ULTOMIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	1.4	0.0	1.8	0.0
Rest of World | STRENSIQ
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	10.3	3.5	16.8	6.7
Rest of World | KANUMA
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	8.9	2.8	10.8	5.4
Product
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	$ 1,444.5	$ 1,202.5	$ 2,889.1	$ 2,342.7