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Investment Securities - Fair Value Measurement by Level (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Investment securities
Other long-term investments	$ 1,377.8	$ 786.8
Debt securities issued by corporations
Investment securities
Fair value investments	992.9	547.4
Mortgage and asset-backed securities
Investment securities
Fair value investments	277.6	218.5
Collateralized loan obligations
Investment securities
Fair value investments	135.0	0.0
Level 2 | Mortgage and asset-backed securities
Investment securities
Fair value investments	277.6	218.5
Level 2 | Collateralized loan obligations
Investment securities
Fair value investments	135.0	0.0
Level 3
Investment securities
Other long-term investments	890.6
Level 3 | Mortgage and asset-backed securities
Investment securities
Fair value investments	0.0	0.0
Level 3 | Collateralized loan obligations
Investment securities
Fair value investments	0.0	0.0
Level 3 | Common equity securities
Investment securities
Fair value investments		0.0	$ 0.1
Level 3 | Other long-term investments
Investment securities
Fair value investments	890.6	614.2	$ 654.0
Fair value measured on a recurring basis
Investment securities
Fair value investments	4,265.3	2,939.0
Other long-term investments	1,377.8	786.8
Fair value measured on a recurring basis | MediaAlpha
Investment securities
Other long-term investments	261.6	802.2
Fair value measured on a recurring basis | U.S. Government and agency obligations
Investment securities
Fair value investments	211.5	176.3
Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	992.9	547.4
Fair value measured on a recurring basis | Municipal obligations
Investment securities
Fair value investments	291.9	265.0
Fair value measured on a recurring basis | Mortgage and asset-backed securities
Investment securities
Fair value investments	277.6	218.5
Fair value measured on a recurring basis | Collateralized loan obligations
Investment securities
Fair value investments	135.0
Fair value measured on a recurring basis | Fixed maturity investments
Investment securities
Fair value investments	1,908.9	1,207.2
Fair value measured on a recurring basis | Short-term investments
Investment securities
Short-term investments, at fair value	465.9	142.8
Fair value measured on a recurring basis | Common equity securities
Investment securities
Fair value investments	251.1
Fair value measured on a recurring basis | Other long-term investments
Investment securities
Fair value investments	895.3	614.2
Fair value measured on a recurring basis | Other long-term investments — NAV
Investment securities
Fair value investments	482.5	172.6
Fair value measured on a recurring basis | Level 1
Investment securities
Fair value investments	939.0	1,121.3
Other long-term investments	0.0	0.0
Fair value measured on a recurring basis | Level 1 | MediaAlpha
Investment securities
Other long-term investments	261.6	802.2
Fair value measured on a recurring basis | Level 1 | U.S. Government and agency obligations
Investment securities
Fair value investments	211.5	176.3
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 1 | Municipal obligations
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 1 | Mortgage and asset-backed securities
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 1 | Collateralized loan obligations
Investment securities
Fair value investments	0.0
Fair value measured on a recurring basis | Level 1 | Fixed maturity investments
Investment securities
Fair value investments	211.5	176.3
Fair value measured on a recurring basis | Level 1 | Short-term investments
Investment securities
Short-term investments, at fair value	465.9	142.8
Fair value measured on a recurring basis | Level 1 | Common equity securities
Investment securities
Fair value investments	0.0
Fair value measured on a recurring basis | Level 1 | Other long-term investments
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 1 | Other long-term investments — NAV
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 2
Investment securities
Fair value investments	1,953.2	1,030.9
Other long-term investments	4.7	0.0
Fair value measured on a recurring basis | Level 2 | MediaAlpha
Investment securities
Other long-term investments	0.0	0.0
Fair value measured on a recurring basis | Level 2 | U.S. Government and agency obligations
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	992.9	547.4
Fair value measured on a recurring basis | Level 2 | Municipal obligations
Investment securities
Fair value investments	291.9	265.0
Fair value measured on a recurring basis | Level 2 | Mortgage and asset-backed securities
Investment securities
Fair value investments	277.6	218.5
Fair value measured on a recurring basis | Level 2 | Collateralized loan obligations
Investment securities
Fair value investments	135.0
Fair value measured on a recurring basis | Level 2 | Fixed maturity investments
Investment securities
Fair value investments	1,697.4	1,030.9
Fair value measured on a recurring basis | Level 2 | Short-term investments
Investment securities
Short-term investments, at fair value	0.0	0.0
Fair value measured on a recurring basis | Level 2 | Common equity securities
Investment securities
Fair value investments	251.1
Fair value measured on a recurring basis | Level 2 | Other long-term investments
Investment securities
Fair value investments	4.7	0.0
Fair value measured on a recurring basis | Level 2 | Other long-term investments — NAV
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 3
Investment securities
Fair value investments	890.6	614.2
Other long-term investments	890.6	614.2
Fair value measured on a recurring basis | Level 3 | MediaAlpha
Investment securities
Other long-term investments	0.0	0.0
Fair value measured on a recurring basis | Level 3 | U.S. Government and agency obligations
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 3 | Municipal obligations
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 3 | Mortgage and asset-backed securities
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 3 | Collateralized loan obligations
Investment securities
Fair value investments	0.0
Fair value measured on a recurring basis | Level 3 | Fixed maturity investments
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 3 | Short-term investments
Investment securities
Short-term investments, at fair value	0.0	0.0
Fair value measured on a recurring basis | Level 3 | Common equity securities
Investment securities
Fair value investments	0.0
Fair value measured on a recurring basis | Level 3 | Other long-term investments
Investment securities
Fair value investments	890.6	614.2
Fair value measured on a recurring basis | Level 3 | Other long-term investments — NAV
Investment securities
Fair value investments	0.0	0.0
Financials | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	264.2	133.9
Financials | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Financials | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	264.2	133.9
Financials | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Consumer | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	178.1	81.9
Consumer | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Consumer | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	178.1	81.9
Consumer | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Technology | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	117.9	66.7
Technology | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Technology | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	117.9	66.7
Technology | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Industrial | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	112.9	66.9
Industrial | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Industrial | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	112.9	66.9
Industrial | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Healthcare | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	112.8	51.5
Healthcare | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Healthcare | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	112.8	51.5
Healthcare | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Utilities | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	70.9	32.3
Utilities | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Utilities | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	70.9	32.3
Utilities | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Communications | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	56.0	44.5
Communications | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Communications | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	56.0	44.5
Communications | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	48.0	35.8
Energy | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Energy | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	48.0	35.8
Energy | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Materials | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	32.1	33.9
Materials | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Materials | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	32.1	33.9
Materials | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations
Investment securities
Fair value investments	$ 0.0	$ 0.0

X

- Definition

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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Amount of long-term investments classified as other.

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Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

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