Fair Value Measurements - Fair Value of Financial Assets by Level (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	$ 1,038.6	$ 1,741.9
Marketable Securities:	6,618.9
Total Financial Assets	7,658.9	7,284.4
Debt Securities, Available-for-Sale, Fair Value to Amortized Cost, after Allowance for Credit Loss [Abstract]
Amortized Cost	6,743.6	5,580.3
Unrealized Gains	17.0	8.7
Unrealized Losses	(3.6)	(13.5)
Fair Value	6,757.0	5,575.5
Marketable securities
Assets, Fair Value Disclosure [Abstract]
Marketable Securities	6,618.9	5,541.1
Commercial paper
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	32.6	48.8
Debt Securities, Available-for-Sale, Fair Value to Amortized Cost, after Allowance for Credit Loss [Abstract]
Amortized Cost	77.0	48.8
Unrealized Gains	0.0	0.0
Unrealized Losses	0.0	0.0
Fair Value	77.0	48.8
U.S. government notes
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	2,235.4	1,921.5
Debt Securities, Available-for-Sale, Fair Value to Amortized Cost, after Allowance for Credit Loss [Abstract]
Amortized Cost	2,280.4	1,954.8
Unrealized Gains	5.3	2.7
Unrealized Losses	(1.4)	(4.6)
Fair Value	2,284.3	1,952.9
Corporate bonds
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	3,219.5	2,593.6
Debt Securities, Available-for-Sale, Fair Value to Amortized Cost, after Allowance for Credit Loss [Abstract]
Amortized Cost	3,210.9	2,595.7
Unrealized Gains	10.1	4.4
Unrealized Losses	(1.5)	(6.5)
Fair Value	3,219.5	2,593.6
Municipal bonds
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	1.5	0.0
Debt Securities, Available-for-Sale, Fair Value to Amortized Cost, after Allowance for Credit Loss [Abstract]
Amortized Cost	1.5	0.0
Unrealized Gains	0.0	0.0
Unrealized Losses	0.0	0.0
Fair Value	1.5	0.0
Agency securities
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	1,129.9	977.2
Debt Securities, Available-for-Sale, Fair Value to Amortized Cost, after Allowance for Credit Loss [Abstract]
Amortized Cost	1,173.8	981.0
Unrealized Gains	1.6	1.6
Unrealized Losses	(0.7)	(2.4)
Fair Value	1,174.7	980.2
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:	900.5	1,707.5
Commercial paper
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	44.4	0.0
U.S. government notes
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	48.9	31.4
Agency securities
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	44.8	3.0
Level I
Assets, Fair Value Disclosure [Abstract]
Cash Equivalents:	949.4	1,738.9
Total Financial Assets	3,186.2	3,661.8
Level I | Marketable securities
Assets, Fair Value Disclosure [Abstract]
Marketable Securities	2,235.4	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:	2,235.4	1,921.5
Level I | Corporate bonds
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	0.0	0.0
Level I | Municipal 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:	900.5	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:	48.9	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:	89.2	3.0
Total Financial Assets	4,472.7	3,622.6
Level II | Marketable securities
Assets, Fair Value Disclosure [Abstract]
Marketable Securities	4,383.5	3,619.6
Level II | Commercial paper
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	32.6	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:	3,219.5	2,593.6
Level II | Municipal bonds
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	1.5	0.0
Level II | Agency securities
Assets, Fair Value Disclosure [Abstract]
Marketable Securities:	1,129.9	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:	44.4	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:	44.8	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 | Municipal 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