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Investment Securities - Investment Gains and Losses For Level 3 (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investment securities
Proceeds from sale and maturity of investments	$ 1,400.0	$ 1,000.0	$ 2,200.0
Level 3
Investment securities
Unrealized investment gains	276.0	181.9	22.6
Other long-term investments | Level 3
Investment securities
Unrealized investment gains	276.0	181.9	$ 22.6
MediaAlpha
Investment securities
Unrealized gain associated with unconsolidated affiliates	$ 278.7	$ 180.0

X

- Definition

Amount of unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in net income (trading).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27357-111563

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- Definition

The cash inflow from sales, maturities, prepayments, calls and collections of all investments, including securities and other assets, having ready marketability and intended by management to be liquidated, if necessary, beyond the current operating cycle.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3179-108585

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- 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.

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- Definition

Amount of unrealized gain (loss) on investment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

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