Investments - Composition of US and Foreign Corporate Securities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Fair Value	$ 21,500.0	$ 19,200.0
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	1,154.1	1,108.8
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	161.4	116.3
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	0.9	2.0
Fixed maturities, including securities pledged, Fair Value	1,314.6	1,223.1
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	1,859.3	1,948.9
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	240.1	133.2
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	10.9	39.6
Fixed maturities, including securities pledged, Fair Value	2,088.5	2,042.5
Industrial and Other Companies
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	7,883.1	6,577.6
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	850.9	559.0
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	6.9	20.7
Fixed maturities, including securities pledged, Fair Value	8,727.1	7,115.9
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	2,715.4	2,527.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	349.8	259.2
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	7.3	6.4
Fixed maturities, including securities pledged, Fair Value	3,057.9	2,780.0
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	396.1	356.9
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	46.5	31.9
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	0.4	0.8
Fixed maturities, including securities pledged, Fair Value	442.2	388.0
U.S. and Foreign Corporate Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	14,008.0	12,519.4
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	1,648.7	1,099.6
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	26.4	69.5
Fixed maturities, including securities pledged, Fair Value	$ 15,630.3	$ 13,549.5