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	$ 1,278.4	$ 1,315.9
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	107.9	81.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	13.9	36.8
Fixed maturities, including securities pledged, Fair Value	1,372.4	1,360.6
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	2,235.6	2,114.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	202.8	166.9
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	10.5	20.2
Fixed maturities, including securities pledged, Fair Value	2,427.9	2,261.4
Industrial and Other Companies
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	8,989.1	8,878.5
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	541.6	423.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	108.0	213.1
Fixed maturities, including securities pledged, Fair Value	9,422.7	9,088.9
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	420.2	440.0
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	30.2	22.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	3.4	9.9
Fixed maturities, including securities pledged, Fair Value	447.0	452.6
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	2,675.9	2,726.5
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	215.0	159.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	17.2	42.3
Fixed maturities, including securities pledged, Fair Value	2,873.7	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	15,599.2	15,475.6
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	1,097.5	853.9
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	153.0	322.3
Fixed maturities, including securities pledged, Fair Value	$ 16,543.7	$ 16,007.2