Investments Composition of US and Foreign Corporate Securities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Fair Value	$ 20,900.0
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	1,112.1	1,108.8
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	166.7	116.3
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	0.4	2.0
Fixed maturities, including securities pledged, Fair Value	1,278.4	1,223.1
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	1,768.1	1,948.9
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	224.6	133.2
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	12.2	39.6
Fixed maturities, including securities pledged, Fair Value	1,980.5	2,042.5
Industrial and Other Companies
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	7,353.3	6,577.6
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	862.5	559.0
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	6.0	20.7
Fixed maturities, including securities pledged, Fair Value	8,209.8	7,115.9
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	2,717.0	2,527.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	367.4	259.2
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	11.0	6.4
Fixed maturities, including securities pledged, Fair Value	3,073.4	2,780.0
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	378.7	356.9
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	46.2	31.9
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	0	0.8
Fixed maturities, including securities pledged, Fair Value	424.9	388.0
U.S. and Foreign Corporate Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	13,329.2	12,519.4
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	1,667.4	1,099.6
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	29.6	69.5
Fixed maturities, including securities pledged, Fair Value	$ 14,967.0	$ 13,549.5