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Fair Value Measurements - Fair Value Option on Consolidated VIEs and Equity Method Investments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Commercial Mortgage Loans of Consolidated VIEs
Fair Value Option, Quantitative Disclosures
Pre-tax gain (loss) due to change in fair value of assets and liabilities for which the fair value option was elected		$ 0.1		$ 0.1
Interest income		$ 0.1		0.3
Real Estate Ventures
Fair Value Option, Quantitative Disclosures
Fair value of assets for which fair value option was elected	$ 27.0		$ 27.0		$ 22.8
Pre-tax gain (loss) due to change in fair value of assets and liabilities for which the fair value option was elected	$ 1.7		$ 4.0	$ 2.3

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- Definition

The fair value of commercial mortgage loans of consolidated variable interest entities and equity method investments for which the fair value option was elected.

+ References

No definition available.

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- Definition

For each line item in the statement of financial position, the amounts of gains and losses from fair value changes included in earnings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14172-108612

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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.

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- Definition

Amount of operating interest income, including, but not limited to, amortization and accretion of premiums and discounts on securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

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