Variable Interest Entities - Unconsolidated VIEs (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	$ 278,484.8	$ 276,087.8
Money Market Funds
Total assets of unconsolidated money market mutual funds	3,900.0	4,000.0
Corporate debt securities \| Available-for-sale
Unconsolidated Variable Interest Entity disclosures
Maximum exposure to loss	221.7	225.7
Corporate debt securities \| Available-for-sale \| Variable Interest Entity
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	242.1	238.2
Residential mortgage-backed pass-through securities \| Available-for-sale
Unconsolidated Variable Interest Entity disclosures
Maximum exposure to loss	2,906.3	2,913.9
Residential mortgage-backed pass-through securities \| Available-for-sale \| Variable Interest Entity
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	3,042.3	2,982.4
Residential mortgage-backed pass-through securities \| Trading
Unconsolidated Variable Interest Entity disclosures
Maximum exposure to loss	220.0	282.3
Residential mortgage-backed pass-through securities \| Trading \| Variable Interest Entity
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	220.0	282.3
Commercial mortgage-backed securities \| Available-for-sale
Unconsolidated Variable Interest Entity disclosures
Maximum exposure to loss	4,719.4	4,746.6
Commercial mortgage-backed securities \| Available-for-sale \| Variable Interest Entity
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	4,871.4	4,850.2
Commercial mortgage-backed securities \| Trading
Unconsolidated Variable Interest Entity disclosures
Maximum exposure to loss	27.6	28.2
Commercial mortgage-backed securities \| Trading \| Variable Interest Entity
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	27.6	28.2
Collateralized debt obligations \| Available-for-sale
Unconsolidated Variable Interest Entity disclosures
Maximum exposure to loss	3,779.4	3,226.7
Collateralized debt obligations \| Available-for-sale \| Variable Interest Entity
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	3,737.2	3,215.3
Collateralized debt obligations \| Trading
Unconsolidated Variable Interest Entity disclosures
Maximum exposure to loss	20.5	20.9
Collateralized debt obligations \| Trading \| Variable Interest Entity
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	20.5	20.9
Other debt obligations \| Available-for-sale
Unconsolidated Variable Interest Entity disclosures
Maximum exposure to loss	7,163.4	8,076.4
Other debt obligations \| Available-for-sale \| Variable Interest Entity
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	7,385.7	8,191.1
Other debt obligations \| Trading
Unconsolidated Variable Interest Entity disclosures
Maximum exposure to loss	10.3	13.2
Other debt obligations \| Trading \| Variable Interest Entity
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	10.3	13.2
Equity securities
Unconsolidated Variable Interest Entity disclosures
Maximum exposure to loss	105.1	123.2
Equity securities \| Variable Interest Entity
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	105.1	123.2
Other investments: Other limited partnership and fund interests
Unconsolidated Variable Interest Entity disclosures
Maximum exposure to loss	1,489.8	1,467.0
Fully secured debt of international real estate funds with recourse to the investment manager, included in maximum loss exposure	128.4	129.1
Other investments: Other limited partnership and fund interests \| Variable Interest Entity
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	$ 956.1	$ 911.9

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Total assets held by money market mutual funds that are not consolidated by the entity as the money market funds are registered under Rule 2a-7 of the Investment Company Act of 1940 or are similar unregistered funds.

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The amount of debt of managed international rest estate funds that is fully secured by assets whose value exceeds the amount of the debt, but also includes recourse to the investment manager, included in the reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

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