Revenue Recognition (Disaggregation of Revenue by Product and Geographical Region) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	$ 1,588.7	$ 1,263.1	$ 4,478.1	$ 3,606.8
SOLIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	1,042.3	990.5	3,040.7	2,933.3
ULTOMIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	289.3	89.9	763.2	168.7
STRENSIQ
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	189.4	154.3	545.9	425.7
ANDEXXA
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	38.9	0.0	38.9	0.0
KANUMA
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	28.4	28.4	88.7	78.1
United States | SOLIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	562.8	496.8	1,672.3	1,456.8
United States | ULTOMIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	170.7	65.1	460.3	143.9
United States | STRENSIQ
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	149.3	118.0	418.1	323.7
United States | ANDEXXA
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	36.2	0.0	36.2	0.0
United States | KANUMA
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	15.8	16.0	47.6	45.1
Europe | SOLIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	254.3	255.5	765.7	800.2
Europe | ULTOMIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	46.6	21.1	112.4	21.1
Europe | STRENSIQ
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	19.3	19.0	61.6	56.0
Europe | ANDEXXA
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	2.7	0.0	2.7	0.0
Europe | KANUMA
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	9.5	6.3	25.4	19.4
Asia Pacific | SOLIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	86.0	118.0	255.5	329.2
Asia Pacific | ULTOMIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	69.6	3.7	186.3	3.7
Asia Pacific | STRENSIQ
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	16.1	14.0	44.7	36.0
Asia Pacific | ANDEXXA
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	0.0	0.0	0.0	0.0
Asia Pacific | KANUMA
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	1.1	1.3	2.9	3.4
Rest of World | SOLIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	139.2	120.2	347.2	347.1
Rest of World | ULTOMIRIS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	2.4	0.0	4.2	0.0
Rest of World | STRENSIQ
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	4.7	3.3	21.5	10.0
Rest of World | ANDEXXA
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	0.0	0.0	0.0	0.0
Rest of World | KANUMA
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	2.0	4.8	12.8	10.2
Product
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	$ 1,588.3	$ 1,263.1	$ 4,477.4	$ 3,605.8