The sum of the periodic adjustments of the differences between securities' face values and purchase prices that are charged against earnings. This is called accretion if the security was purchased at a discount and amortization if it was purchased at premium. As a noncash item, this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Securities, Realized and Unrealized Gain (Loss)

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Investment Securities - Rollforward of Fair Value Measurements by Level (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	$ 2,796.2	$ 2,746.0
Net realized and unrealized gains (losses)	(227.8)	302.4
Amortization/Accretion	(6.0)	(3.0)
Purchases	1,463.8	563.6
Sales	(645.4)	(1,075.6)
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	3,594.4	2,533.4
Ark
Debt and Equity Securities, FV-NI [Line Items]
Purchases	213.6
Short-term investments, at fair value	396.5		$ 0.0
Third Party Capital Providers
Debt and Equity Securities, FV-NI [Line Items]
Realized and unrealized gains (losses)	6.5
Level 1 Investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	978.5	780.0
Net realized and unrealized gains (losses)	(327.6)	29.2
Amortization/Accretion	(0.1)	0.0
Purchases	130.9	128.8
Sales	(257.7)	(642.7)
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	524.0	295.3
Level 1 Investments \| Ark
Debt and Equity Securities, FV-NI [Line Items]
Purchases	0.0
Level 2 Investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	1,030.9	1,109.6
Net realized and unrealized gains (losses)	(13.1)	12.5
Amortization/Accretion	(5.9)	(3.0)
Purchases	989.7	316.2
Sales	(284.9)	(368.5)
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	1,784.9	1,066.8
Level 2 Investments \| Ark
Debt and Equity Securities, FV-NI [Line Items]
Purchases	68.2
Common equity securities \| Level 3 Investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		0.1
Net realized and unrealized gains (losses)		0.0
Amortization/Accretion		0.0
Purchases		0.0
Transfers in		0.0
Transfers out		0.0
Ending balance		0.1
Other long-term investments \| Level 3 Investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	614.2	654.0
Net realized and unrealized gains (losses)	79.0	275.6
Amortization/Accretion	0.0	0.0
Purchases	143.6	78.8
Sales	(31.9)	(8.1)
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	814.5	1,000.3
Other long-term investments \| Level 3 Investments \| Ark
Debt and Equity Securities, FV-NI [Line Items]
Purchases	9.6
Hedge and private equity funds included in other long-term investments \| NAV
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	172.6	202.3
Net realized and unrealized gains (losses)	33.9	(14.9)
Amortization/Accretion	0.0	0.0
Purchases	199.6	39.8
Sales	(70.9)	(56.3)
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	471.0	170.9
Hedge and private equity funds included in other long-term investments \| NAV \| Ark
Debt and Equity Securities, FV-NI [Line Items]
Purchases	135.8
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	$ 609.5	571.8	$ 142.9	$ 201.2
Realized and unrealized gains (losses)		$ 0.4