Fair Value Measurements - Fair Value of Financial Assets by Level (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	$ 1,133.7	$ 1,741.9
Marketable Securities:	6,304.5
Total Financial Assets	7,439.6	7,284.4
Debt Securities, Available-for-Sale, Fair Value to Amortized Cost, after Allowance for Credit Loss [Abstract]
Amortized Cost	6,646.4	5,580.3
Unrealized Gains	14.3	8.7
Unrealized Losses	(5.2)	(13.5)
Fair Value	6,655.5	5,575.5
Marketable securities
Assets, Fair Value Disclosure [Abstract]
Marketable Securities	6,304.5	5,541.1
Commercial paper
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	43.3	48.8
Debt Securities, Available-for-Sale, Fair Value to Amortized Cost, after Allowance for Credit Loss [Abstract]
Amortized Cost	105.4	48.8
Unrealized Gains	0.0	0.0
Unrealized Losses	0.0	0.0
Fair Value	105.4	48.8
U.S. government notes
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	2,094.8	1,921.5
Debt Securities, Available-for-Sale, Fair Value to Amortized Cost, after Allowance for Credit Loss [Abstract]
Amortized Cost	2,380.1	1,954.8
Unrealized Gains	5.2	2.7
Unrealized Losses	(1.6)	(4.6)
Fair Value	2,383.7	1,952.9
Corporate bonds
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	3,111.8	2,593.6
Debt Securities, Available-for-Sale, Fair Value to Amortized Cost, after Allowance for Credit Loss [Abstract]
Amortized Cost	3,107.2	2,595.7
Unrealized Gains	7.3	4.4
Unrealized Losses	(2.7)	(6.5)
Fair Value	3,111.8	2,593.5
Agency securities
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	1,054.6	977.2
Debt Securities, Available-for-Sale, Fair Value to Amortized Cost, after Allowance for Credit Loss [Abstract]
Amortized Cost	1,053.7	981.0
Unrealized Gains	1.8	1.6
Unrealized Losses	(0.9)	(2.4)
Fair Value	1,054.6	980.3
Money market funds - restricted
Assets, Fair Value Disclosure [Abstract]
Money market funds - restricted	1.4	1.4
Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	782.7	1,707.5
Commercial paper
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	62.1	0.0
U.S. government notes
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	288.9	31.4
Agency securities
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	0.0	3.0
Level I
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	1,071.6	1,738.9
Total Financial Assets	3,167.8	3,661.8
Level I | Marketable securities
Assets, Fair Value Disclosure [Abstract]
Marketable Securities	2,094.8	1,921.5
Level I | Commercial paper
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	0.0	0.0
Level I | U.S. government notes
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	2,094.8	1,921.5
Level I | Corporate bonds
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	0.0	0.0
Level I | Agency securities
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	0.0	0.0
Level I | Money market funds - restricted
Assets, Fair Value Disclosure [Abstract]
Money market funds - restricted	1.4	1.4
Level I | Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	782.7	1,707.5
Level I | Commercial paper
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	0.0	0.0
Level I | U.S. government notes
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	288.9	31.4
Level I | Agency securities
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	0.0	0.0
Level II
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	62.1	3.0
Total Financial Assets	4,271.8	3,622.6
Level II | Marketable securities
Assets, Fair Value Disclosure [Abstract]
Marketable Securities	4,209.7	3,619.6
Level II | Commercial paper
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	43.3	48.8
Level II | U.S. government notes
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	0.0	0.0
Level II | Corporate bonds
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	3,111.8	2,593.6
Level II | Agency securities
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	1,054.6	977.2
Level II | Money market funds - restricted
Assets, Fair Value Disclosure [Abstract]
Money market funds - restricted	0.0	0.0
Level II | Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	0.0	0.0
Level II | Commercial paper
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	62.1	0.0
Level II | U.S. government notes
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	0.0	0.0
Level II | Agency securities
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	0.0	3.0
Level III
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	0.0	0.0
Total Financial Assets	0.0	0.0
Level III | Marketable securities
Assets, Fair Value Disclosure [Abstract]
Marketable Securities	0.0	0.0
Level III | Commercial paper
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	0.0	0.0
Level III | U.S. government notes
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	0.0	0.0
Level III | Corporate bonds
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	0.0	0.0
Level III | Agency securities
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	0.0	0.0
Level III | Money market funds - restricted
Assets, Fair Value Disclosure [Abstract]
Money market funds - restricted	0.0	0.0
Level III | Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	0.0	0.0
Level III | Commercial paper
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	0.0	0.0
Level III | U.S. government notes
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	0.0	0.0
Level III | Agency securities
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	$ 0.0	$ 0.0