Variable Interest Entities - Unconsolidated VIEs (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Money Market Funds
Total assets of unconsolidated money market mutual funds	$ 700.0	$ 800.0
Other investments: Other limited partnership and fund interests
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	719.2	654.6
Maximum exposure to loss	1,220.0	1,127.8
Available-for-sale \| Corporate debt securities
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	370.2	368.4
Maximum exposure to loss	303.1	298.6
Available-for-sale \| Residential mortgage-backed pass-through securities
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	2,839.9	2,834.7
Maximum exposure to loss	2,807.4	2,798.0
Available-for-sale \| Commercial mortgage-backed securities
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	3,862.9	4,096.5
Maximum exposure to loss	3,903.8	4,153.2
Available-for-sale \| Collateralized debt obligations
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	987.9	758.6
Maximum exposure to loss	1,006.9	780.1
Available-for-sale \| Other debt obligations
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	5,246.9	5,036.1
Maximum exposure to loss	5,252.1	5,048.9
Trading \| Residential mortgage-backed pass-through securities
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	18.8	19.9
Maximum exposure to loss	18.8	19.9
Trading \| Commercial mortgage-backed securities
Unconsolidated Variable Interest Entity disclosures
Asset carrying value		1.9
Maximum exposure to loss		1.9
Trading \| Collateralized debt obligations
Unconsolidated Variable Interest Entity disclosures
Asset carrying value		10.6
Maximum exposure to loss		10.6
Trading \| Equity securities
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	76.5	68.3
Maximum exposure to loss	$ 76.5	$ 68.3

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- Definition

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.

+ References

No definition available.

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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

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph b
-URI http://asc.fasb.org/extlink&oid=64801991&loc=d3e5728-111685

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=64801991&loc=d3e5747-111685

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- Definition

Carrying amount of the assets in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), where the reporting entity is not the VIE's primary beneficiary.

+ References

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