Investment Securities (Fair Value Measurement by Level) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) Investment	Dec. 31, 2018 USD ($) Investment	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Investment securities
Percentage of investments recorded at fair value	71.00%	87.00%
Fair value investments	$ 2,746.0	$ 2,328.7		$ 3,208.3
Debt securities issued by corporations
Investment securities
Fair value investments	467.2	510.5
Mortgage and asset-backed securities
Investment securities
Fair value investments	$ 209.0	$ 133.5
Common Stock
Investment securities
Number of investments | Investment	2	2
Level 1
Investment securities
Fair value investments	$ 780.0	$ 842.6	$ 842.6	890.4
Level 2
Investment securities
Fair value investments	1,109.6	1,160.5	1,160.5	2,105.4
Level 2 | Mortgage and asset-backed securities
Investment securities
Fair value investments	209.0	133.5
Level 3 Inputs | Mortgage and asset-backed securities
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs | Common equity securities
Investment securities
Fair value investments	0.1	0.0
Level 3 Inputs | Other long-term investments
Investment securities
Fair value investments	654.0	138.7	138.7	77.2
Fair Value Measured at Net Asset Value Per Share | Hedge Funds and Private Equity Funds
Investment securities
Fair value investments	202.3	186.9		$ 135.3
Fair value measured on a recurring basis
Investment securities
Fair value investments	2,947.2	2,542.9
Fair value measured on a recurring basis | U.S. Government and agency obligations
Investment securities
Fair value investments	232.5	153.2
Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	467.2	510.5
Fair value measured on a recurring basis | Mortgage and asset-backed securities
Investment securities
Fair value investments	209.0	133.5
Fair value measured on a recurring basis | Municipal obligations
Investment securities
Fair value investments	297.1	280.3
Fair value measured on a recurring basis | Fixed maturity investments
Investment securities
Fair value investments	1,205.8	1,077.5
Fair value measured on a recurring basis | Short-term investments
Investment securities
Short-term investments, at fair value	201.2	214.2
Fair value measured on a recurring basis | Other long-term investments
Investment securities
Fair value investments	654.0	138.7
Fair value measured on a recurring basis | Level 1
Investment securities
Fair value investments	969.4	1,047.0
Fair value measured on a recurring basis | Level 1 | U.S. Government and agency obligations
Investment securities
Fair value investments	232.5	153.2
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 | Mortgage and asset-backed securities
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 | Fixed maturity investments
Investment securities
Fair value investments	232.5	153.2
Fair value measured on a recurring basis | Level 1 | Short-term investments
Investment securities
Short-term investments, at fair value	189.4	204.4
Fair value measured on a recurring basis | Level 1 | Common equity securities
Investment securities
Fair value investments	547.5
Fair value measured on a recurring basis | Level 1 | Other long-term investments
Investment securities
Fair value investments	0.0		$ 2,328.7
Fair value measured on a recurring basis | Level 2
Investment securities
Fair value investments	1,121.4	1,170.3
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	467.2	510.5
Fair value measured on a recurring basis | Level 2 | Mortgage and asset-backed securities
Investment securities
Fair value investments	209.0	133.5
Fair value measured on a recurring basis | Level 2 | Municipal obligations
Investment securities
Fair value investments	297.1	280.3
Fair value measured on a recurring basis | Level 2 | Fixed maturity investments
Investment securities
Fair value investments	973.3	924.3
Fair value measured on a recurring basis | Level 2 | Short-term investments
Investment securities
Short-term investments, at fair value	11.8	9.8
Fair value measured on a recurring basis | Level 2 | Common equity securities
Investment securities
Fair value investments	136.3
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 3 Inputs
Investment securities
Fair value investments	654.1	138.7
Fair value measured on a recurring basis | Level 3 Inputs | U.S. Government and agency obligations
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 3 Inputs | Mortgage and asset-backed securities
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 3 Inputs | Municipal obligations
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 3 Inputs | Fixed maturity investments
Investment securities
Fair value investments	0.0	0.0
Fair value measured on a recurring basis | Level 3 Inputs | Short-term investments
Investment securities
Short-term investments, at fair value	0.0	0.0
Fair value measured on a recurring basis | Level 3 Inputs | Other long-term investments
Investment securities
Fair value investments	654.0	138.7
Fair value measured on a recurring basis | Fair Value Measured at Net Asset Value Per Share | Other long-term investments
Investment securities
Fair value investments		186.9
Financials | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	144.8	143.4
Financials | Fair value measured on a recurring basis | Common equity securities
Investment securities
Fair value investments		13.5
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 1 | Common equity securities
Investment securities
Fair value investments		13.5
Financials | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	144.8	143.4
Financials | Fair value measured on a recurring basis | Level 2 | Common equity securities
Investment securities
Fair value investments		0.0
Financials | Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations
Investment securities
Fair value investments		0.0
Financials | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities
Investment securities
Fair value investments		0.0
Industrial | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	59.0	47.6
Industrial | Fair value measured on a recurring basis | Common equity securities
Investment securities
Fair value investments		11.4
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 1 | Common equity securities
Investment securities
Fair value investments		11.4
Industrial | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	59.0	47.6
Industrial | Fair value measured on a recurring basis | Level 2 | Common equity securities
Investment securities
Fair value investments		0.0
Industrial | Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Industrial | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities
Investment securities
Fair value investments		0.0
Healthcare | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	52.6	55.0
Healthcare | Fair value measured on a recurring basis | Common equity securities
Investment securities
Fair value investments		14.0
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 1 | Common equity securities
Investment securities
Fair value investments		14.0
Healthcare | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	52.6	55.0
Healthcare | Fair value measured on a recurring basis | Level 2 | Common equity securities
Investment securities
Fair value investments		0.0
Healthcare | Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Healthcare | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities
Investment securities
Fair value investments		0.0
Consumer | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	50.9	68.5
Consumer | Fair value measured on a recurring basis | Common equity securities
Investment securities
Fair value investments		6.2
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 1 | Common equity securities
Investment securities
Fair value investments		6.2
Consumer | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	50.9	68.5
Consumer | Fair value measured on a recurring basis | Level 2 | Common equity securities
Investment securities
Fair value investments		0.0
Consumer | Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Consumer | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities
Investment securities
Fair value investments		0.0
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	44.9	57.6
Energy | Fair value measured on a recurring basis | Common equity securities
Investment securities
Fair value investments		4.1
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 1 | Common equity securities
Investment securities
Fair value investments		4.1
Energy | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	44.9	57.6
Energy | Fair value measured on a recurring basis | Level 2 | Common equity securities
Investment securities
Fair value investments		0.0
Energy | Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Energy | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities
Investment securities
Fair value investments		0.0
Technology | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	41.2	60.5
Technology | Fair value measured on a recurring basis | Common equity securities
Investment securities
Fair value investments		7.4
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 1 | Common equity securities
Investment securities
Fair value investments		7.4
Technology | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	41.2	60.5
Technology | Fair value measured on a recurring basis | Level 2 | Common equity securities
Investment securities
Fair value investments		0.0
Technology | Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Technology | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities
Investment securities
Fair value investments		0.0
Communications | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	31.3	31.8
Communications | Fair value measured on a recurring basis | Common equity securities
Investment securities
Fair value investments		12.7
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 1 | Common equity securities
Investment securities
Fair value investments		12.7
Communications | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	31.3	31.8
Communications | Fair value measured on a recurring basis | Level 2 | Common equity securities
Investment securities
Fair value investments		0.0
Communications | Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Communications | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities
Investment securities
Fair value investments		0.0
Utilities | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments	25.0	19.8
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	25.0	19.8
Utilities | Fair value measured on a recurring basis | Level 3 Inputs | 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	17.5	26.3
Materials | Fair value measured on a recurring basis | Common equity securities
Investment securities
Fair value investments		3.1
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 1 | Common equity securities
Investment securities
Fair value investments		3.1
Materials | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments	17.5	26.3
Materials | Fair value measured on a recurring basis | Level 2 | Common equity securities
Investment securities
Fair value investments		0.0
Materials | Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	0.0
Materials | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities
Investment securities
Fair value investments		0.0
Exchange traded funds | Fair value measured on a recurring basis | Common equity securities
Investment securities
Fair value investments	536.4	675.3
Exchange traded funds | Fair value measured on a recurring basis | Level 1 | Common equity securities
Investment securities
Fair value investments	521.6	617.0
Exchange traded funds | Fair value measured on a recurring basis | Level 2 | Common equity securities
Investment securities
Fair value investments	14.8	58.3
Exchange traded funds | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities
Investment securities
Fair value investments	0.0	0.0
Other | Fair value measured on a recurring basis | Common equity securities
Investment securities
Fair value investments	147.5	177.9
Other | Fair value measured on a recurring basis | Level 1 | Common equity securities
Investment securities
Fair value investments	25.9	0.0
Other | Fair value measured on a recurring basis | Level 2 | Common equity securities
Investment securities
Fair value investments	121.5	177.9
Other | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities
Investment securities
Fair value investments	0.1	0.0
Common equity securities | Fair value measured on a recurring basis | Common equity securities
Investment securities
Fair value investments	683.9	925.6
Common equity securities | Fair value measured on a recurring basis | Level 1 | Common equity securities
Investment securities
Fair value investments		689.4
Common equity securities | Fair value measured on a recurring basis | Level 2 | Common equity securities
Investment securities
Fair value investments		236.2
Common equity securities | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities
Investment securities
Fair value investments	$ 0.1	$ 0.0