Investments - Composition of US and Foreign Corporate Securities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	$ 1,288.3	$ 1,315.9
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	137.6	81.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	5.2	36.8
Fixed maturities, including securities pledged, Fair Value	1,420.7	1,360.6
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	2,247.4	2,114.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	227.8	166.9
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	2.6	20.2
Fixed maturities, including securities pledged, Fair Value	2,472.6	2,261.4
Industrial and Other Companies
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	8,923.3	8,878.5
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	661.3	423.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	48.8	213.1
Fixed maturities, including securities pledged, Fair Value	9,535.8	9,088.9
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	416.1	440.0
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	37.3	22.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	1.0	9.9
Fixed maturities, including securities pledged, Fair Value	452.4	452.6
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	2,650.1	2,726.5
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	266.7	159.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	6.8	42.3
Fixed maturities, including securities pledged, Fair Value	2,910.0	2,843.7
U.S. and Foreign Corporate Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	15,525.2	15,475.6
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	1,330.7	853.9
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	64.4	322.3
Fixed maturities, including securities pledged, Fair Value	$ 16,791.5	$ 16,007.2