Fair Value Measurements - Fair Value Hierarchy (Details) - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Assets
Fair Value	$ 697,600,000	$ 6,600,000	$ 5,900,000
Total	805,100,000	6,600,000	5,900,000
Fair value assets level 1 to level 2 transfers	0	0	0
Fair value assets level 2 to level 1 transfers	0	0	0
Fair value assets transferred into (out of) level 3	0	0	0
Level 1
Assets
Fair Value	0	0	0
Total	0	0	0
Level 2
Assets
Fair Value	697,600,000	6,600,000	5,900,000
Total	805,100,000	6,600,000	5,900,000
Level 3
Assets
Fair Value	0	0	0
Total	$ 0	$ 0	$ 0

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

+ References

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