Investments - Composition of US and Foreign Corporate Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Communications
Available-for-sale Securities, Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	$ 3,966.9	$ 4,016.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	403.4	293.0
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	29.2	73.4
Fixed maturities, including securities pledged	4,341.1	4,235.8
Financial
Available-for-sale Securities, Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	6,990.9	6,640.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	675.5	566.6
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	42.7	72.6
Fixed maturities, including securities pledged	7,623.7	7,134.7
Industrial and Other Companies
Available-for-sale Securities, Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	29,959.3	29,303.1
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	1,964.8	1,524.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	293.2	564.5
Fixed maturities, including securities pledged	31,630.9	30,263.1
Utilities
Available-for-sale Securities, Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	9,165.5	9,200.6
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	778.4	570.0
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	70.8	142.2
Fixed maturities, including securities pledged	9,873.1	9,628.4
Transportation
Available-for-sale Securities, Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	1,436.1	1,467.1
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	116.4	84.6
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	14.4	30.0
Fixed maturities, including securities pledged	1,538.1	1,521.7
U.S. and Foreign Corporate Securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	51,518.7	50,627.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	3,938.5	3,038.7
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	450.3	882.7
Fixed maturities, including securities pledged	$ 55,006.9	$ 52,783.7