Investments - Composition of US and Foreign Corporate Securities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	$ 1,318.7	$ 1,154.1
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	89.8	161.4
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	29.4	0.9
Fixed maturities, including securities pledged, Fair Value	1,379.1	1,314.6
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	1,844.1	1,859.3
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	154.3	240.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	22.4	10.9
Fixed maturities, including securities pledged, Fair Value	1,976.0	2,088.5
Industrial and Other Companies
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	8,566.0	7,883.1
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	439.0	850.9
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	188.1	6.9
Fixed maturities, including securities pledged, Fair Value	8,816.9	8,727.1
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	2,697.6	2,715.4
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	186.2	349.8
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	28.4	7.3
Fixed maturities, including securities pledged, Fair Value	2,855.4	3,057.9
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	440.7	396.1
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	25.2	46.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	8.2	0.4
Fixed maturities, including securities pledged, Fair Value	457.7	442.2
U.S. and Foreign Corporate Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	14,867.1	14,008.0
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	894.5	1,648.7
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	276.5	26.4
Fixed maturities, including securities pledged, Fair Value	$ 15,485.1	$ 15,630.3