Schdeule of Level 3 Assets and Liabilities by Pricing Source (Details) (Level 3, USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Embedded derivatives
Level 3 Assets and Liabilities by Pricing Source:
Total liabilities Internal	$ 160.3	$ 87.8
Total liabilities External	0	0
Total liabilities	160.3	87.8
Total liabilities
Level 3 Assets and Liabilities by Pricing Source:
Total liabilities Internal	160.3	87.8
Total liabilities External	0	0
Total liabilities	160.3	87.8
U.S. government and agency
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	80.2	58.8
Total assets External	0	0
Total assets	80.2	58.8	25.9
State and political subdivision
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	65.9	45.4
Total assets External	127.4	88.4
Total assets	193.3	133.8	116.1
Foreign government
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	0	0
Total assets External	16.1	3.3
Total assets	16.1	3.3	8.0
Corporate
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	1,053.3	874.8
Total assets External	363.9	217.1
Total assets	1,417.2	1,091.9	797.9
CMBS
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	0	0
Total assets External	64.6	31.9
Total assets	64.6	31.9	19.3
RMBS
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	0	0
Total assets External	151.6	176.1
Total assets	151.6	176.1	223.3
Collateralized debt obligation (“CDO”) / collateralized loan obligation (“CLO”)
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	0	0
Total assets External	83.5	70.9
Total assets	83.5	70.9	56.8
Other asset-backed (“ABS”)
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	9.8	11.3
Total assets External	68.1	71.0
Total assets	77.9	82.3	80.1
Total available-for-sale debt securities
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	1,209.2	990.3
Total assets External	875.2	658.7
Total assets	2,084.4	1,649.0	1,327.4
Available-for-sale equity securities
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	0	0
Total assets External	28.7	11.2
Total assets	28.7	11.2	3.6
Fair value investments
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	1.0	0.8
Total assets External	32.7	34.8
Total assets	33.7	35.6	22.7
Total assets
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	1,210.2	991.1
Total assets External	936.6	704.7
Total assets	$ 2,146.8	$ 1,695.8	$ 1,353.7