Investments - Composition of US and Foreign Corporate Securities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	$ 1,327.7	$ 1,154.1
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	78.9	161.4
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	40.9	0.9
Fixed maturities, including securities pledged, Fair Value	1,365.7	1,314.6
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	1,999.8	1,859.3
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	161.0	240.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	22.0	10.9
Fixed maturities, including securities pledged, Fair Value	2,138.8	2,088.5
Industrial and Other Companies
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	8,896.5	7,883.1
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	440.0	850.9
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	206.6	6.9
Fixed maturities, including securities pledged, Fair Value	9,129.9	8,727.1
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	2,628.0	2,715.4
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	167.1	349.8
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	37.8	7.3
Fixed maturities, including securities pledged, Fair Value	2,757.3	3,057.9
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	435.2	396.1
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	24.0	46.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	9.4	0.4
Fixed maturities, including securities pledged, Fair Value	449.8	442.2
U.S. and Foreign Corporate Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	15,287.2	14,008.0
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	871.0	1,648.7
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	316.7	26.4
Fixed maturities, including securities pledged, Fair Value	$ 15,841.5	$ 15,630.3