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Investment Securities - Fair Value Measurement by Level (Details) $ in Millions	Dec. 31, 2020 USD ($) fund	Dec. 31, 2019 USD ($) Investment	Dec. 31, 2018 USD ($)
Investment securities
Fair value investments	$ 2,796.2	$ 2,746.0	$ 2,328.7
Other long-term investments	$ 786.8
Number of investments | fund	14
Debt securities issued by corporations
Investment securities
Fair value investments	$ 547.4	467.2
Mortgage and asset-backed securities
Investment securities
Fair value investments	218.5	209.0
Equity Securities
Investment securities
Fair value investments		$ 0.1	0.0
Common equity securities
Investment securities
Number of investments | Investment		2
Level 1
Investment securities
Fair value investments	978.5	$ 780.0	842.6
Level 2
Investment securities
Fair value investments	1,030.9	1,109.6	1,160.5
Level 2 | Mortgage and asset-backed securities
Investment securities
Fair value investments	218.5	209.0
Level 3
Investment securities
Other long-term investments	614.2
Level 3 | Mortgage and asset-backed securities
Investment securities
Fair value investments	0.0	0.0
Level 3 | Equity Securities
Investment securities
Fair value investments	0.0	0.1
Level 3 | Other long-term investments
Investment securities
Fair value investments	614.2	654.0	138.7
Fair value measured on a recurring basis
Investment securities
Fair value investments	2,939.1	2,947.2
Other long-term investments	786.8	676.3
Fair value measured on a recurring basis | MediaAlpha
Investment securities
Other long-term investments	802.2	180.0
Fair value measured on a recurring basis | U.S. Government and agency obligations
Investment securities
Fair value investments	176.3	232.5
Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	547.4	467.2
Fair value measured on a recurring basis | Municipal obligations
Investment securities
Fair value investments	265.0	297.1
Fair value measured on a recurring basis | Mortgage and asset-backed securities
Investment securities
Fair value investments	218.5	209.0
Fair value measured on a recurring basis | Fixed maturity investments
Investment securities
Fair value investments	1,207.2	1,205.8
Fair value measured on a recurring basis | Short-term investments
Investment securities
Short-term investments, at fair value	142.9	201.2	$ 214.2
Fair value measured on a recurring basis | Other long-term investments
Investment securities
Fair value investments	614.2	474.0
Fair value measured on a recurring basis | Other long-term investments — NAV
Investment securities
Fair value investments	172.6	202.3
Fair value measured on a recurring basis | Level 1
Investment securities
Fair value investments	1,121.4	969.4
Other long-term investments	0.0	0.0
Fair value measured on a recurring basis | Level 1 | MediaAlpha
Investment securities
Other long-term investments	802.2	0.0
Fair value measured on a recurring basis | Level 1 | U.S. Government and agency obligations
Investment securities
Fair value investments	176.3	232.5
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 | Fixed maturity investments
Investment securities
Fair value investments	176.3	232.5
Fair value measured on a recurring basis | Level 1 | Short-term investments
Investment securities
Short-term investments, at fair value	142.9	189.4
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	1,030.9	1,121.4
Other long-term investments	0.0	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	547.4	467.2
Fair value measured on a recurring basis | Level 2 | Municipal obligations
Investment securities
Fair value investments	265.0	297.1
Fair value measured on a recurring basis | Level 2 | Mortgage and asset-backed securities
Investment securities
Fair value investments	218.5	209.0
Fair value measured on a recurring basis | Level 2 | Fixed maturity investments
Investment securities
Fair value investments	1,030.9	973.3
Fair value measured on a recurring basis | Level 2 | Short-term investments
Investment securities
Short-term investments, at fair value	0.0	11.8
Fair value measured on a recurring basis | Level 2 | Other long-term investments
Investment securities
Fair value investments	0.0	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	614.2	654.1
Other long-term investments	614.2	474.0
Fair value measured on a recurring basis | Level 3 | MediaAlpha
Investment securities
Other long-term investments	0.0	180.0
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 | 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 | Other long-term investments
Investment securities
Fair value investments	614.2	474.0
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	133.9	144.8
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	133.9	144.8
Financials | 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	66.9	59.0
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	66.9	59.0
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	51.5	52.6
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	51.5	52.6
Healthcare | 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	81.9	50.9
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	81.9	50.9
Consumer | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	35.8	44.9
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	35.8	44.9
Energy | 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	66.7	41.2
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	66.7	41.2
Technology | 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	44.5	31.3
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	44.5	31.3
Communications | 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	32.3	25.0
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	32.3	25.0
Utilities | 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	33.9	17.5
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	33.9	17.5
Materials | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations
Investment securities
Fair value investments	$ 0.0	0.0
Exchange traded funds | Fair value measured on a recurring basis | Equity Securities
Investment securities
Fair value investments		536.4
Exchange traded funds | Fair value measured on a recurring basis | Level 1 | Equity Securities
Investment securities
Fair value investments		521.6
Exchange traded funds | Fair value measured on a recurring basis | Level 2 | Equity Securities
Investment securities
Fair value investments		14.8
Exchange traded funds | Fair value measured on a recurring basis | Level 3 | Equity Securities
Investment securities
Fair value investments		0.0
Other | Fair value measured on a recurring basis | Equity Securities
Investment securities
Fair value investments		147.5
Other | Fair value measured on a recurring basis | Level 1 | Equity Securities
Investment securities
Fair value investments		25.9
Other | Fair value measured on a recurring basis | Level 2 | Equity Securities
Investment securities
Fair value investments		121.5
Other | Fair value measured on a recurring basis | Level 3 | Equity Securities
Investment securities
Fair value investments		0.1
Equity Securities | Fair value measured on a recurring basis | Equity Securities
Investment securities
Fair value investments		683.9
Equity Securities | Fair value measured on a recurring basis | Level 1 | Equity Securities
Investment securities
Fair value investments		547.5
Equity Securities | Fair value measured on a recurring basis | Level 2 | Equity Securities
Investment securities
Fair value investments		136.3
Equity Securities | Fair value measured on a recurring basis | Level 3 | Equity Securities
Investment securities
Fair value investments		$ 0.1

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- 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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