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Investment Securities - Investment in MediaAlpha textual (Details) - Marketing Technology (MediaAlpha) - USD ($) $ / shares in Units, $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Subsidiary, Sale of Stock [Line Items]
Share price (in usd per share)	$ 18.68	$ 39.07
Investment owned, balance	$ 316.4	$ 802.2

X

- Definition

For investments which are quantified by principal amount, the principle balance held at close of period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 210
-Section 50
-Paragraph 6
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=99383244&loc=d3e12121-115841

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-12B(Column B))
-URI http://asc.fasb.org/extlink&oid=120401592&loc=d3e611197-123010

+ Details

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Balance Type:	debit
Period Type:	instant

X

- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	instant

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Period Type:	duration

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type: