Investment Securities (Fair Value Measurement by Level) (Details) $ in Millions	Jun. 30, 2020 USD ($) Investment	Dec. 31, 2019 USD ($) Investment	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,656.9	$ 2,746.0	$ 2,483.0	$ 2,328.7
Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	665.1	780.0	684.5	842.6
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 1,093.6	$ 1,109.6	1,134.1	1,160.5
Common Stock
Debt and Equity Securities, FV-NI [Line Items]
Number of Investments | Investment	2	2
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 505.8	$ 467.2
Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	220.6	209.0
Mortgage-backed and asset-backed securities | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	220.6	209.0
Mortgage-backed and asset-backed securities | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	117.6	201.2	187.1	214.2
Common equity securities | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.1	0.1
Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	740.1	654.0	$ 458.0	$ 138.7
Fair value measured on a recurring basis
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	2,774.5	2,947.2
Fair value measured on a recurring basis | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	779.6	969.4
Fair value measured on a recurring basis | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,096.7	1,121.4
Fair value measured on a recurring basis | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	740.2	654.1
Fair value measured on a recurring basis | U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	200.4	232.5
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	200.4	232.5
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	505.8	467.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	505.8	467.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	151.9	144.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	151.9	144.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	43.3	52.6
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	43.3	52.6
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	61.1	59.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	61.1	59.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	57.6	41.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	57.6	41.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	34.3	44.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	34.3	44.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	55.6	50.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	55.6	50.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	40.9	31.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	40.9	31.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	29.0	25.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	29.0	25.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	32.1	17.5
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	32.1	17.5
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	220.6	209.0
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	220.6	209.0
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	264.8	297.1
Fair value measured on a recurring basis | Municipal obligations | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Municipal obligations | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	264.8	297.1
Fair value measured on a recurring basis | Municipal obligations | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,191.6	1,205.8
Fair value measured on a recurring basis | Fixed maturity investments | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	200.4	232.5
Fair value measured on a recurring basis | Fixed maturity investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	991.2	973.3
Fair value measured on a recurring basis | Fixed maturity investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	117.6	201.2
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	114.5	189.4
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	3.1	11.8
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	0.0	0.0
Fair value measured on a recurring basis | Common equity securities | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	464.7
Fair value measured on a recurring basis | Common equity securities | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	102.4
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	442.4	536.4
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	442.4	521.6
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	14.8
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Common equity securities | Other
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	124.8	147.5
Fair value measured on a recurring basis | Common equity securities | Other | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	22.3	25.9
Fair value measured on a recurring basis | Common equity securities | Other | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	102.4	121.5
Fair value measured on a recurring basis | Common equity securities | Other | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.1	0.1
Fair value measured on a recurring basis | Common equity securities | Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	567.2	683.9
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		547.5
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments		136.3
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.1	0.1
Fair value measured on a recurring basis | Other long-term investments | Other Long-Term Investment NAV [Member]
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	158.0	202.3
Fair value measured on a recurring basis | Other long-term investments | Other Long-Term Investment NAV [Member] | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0
Fair value measured on a recurring basis | Other long-term investments | Other Long-Term Investment NAV [Member] | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0
Fair value measured on a recurring basis | Other long-term investments | Other Long-Term Investment NAV [Member] | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0
Fair value measured on a recurring basis | Other long-term investments | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	740.1	654.0
Fair value measured on a recurring basis | Other long-term investments | Other long-term investments | Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0
Fair value measured on a recurring basis | Other long-term investments | Other long-term investments | Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Other long-term investments | Other long-term investments | Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 740.1	$ 654.0