Investments - Composition of US and Foreign Corporate Securities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	$ 1,254.6	$ 1,315.9
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	122.0	81.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	7.4	36.8
Fixed maturities, including securities pledged, Fair Value	1,369.2	1,360.6
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	2,286.8	2,114.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	199.4	166.9
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	4.5	20.2
Fixed maturities, including securities pledged, Fair Value	2,481.7	2,261.4
Industrial and Other Companies
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	8,962.5	8,878.5
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	550.8	423.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	70.7	213.1
Fixed maturities, including securities pledged, Fair Value	9,442.6	9,088.9
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	438.1	440.0
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	33.0	22.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	2.7	9.9
Fixed maturities, including securities pledged, Fair Value	468.4	452.6
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	2,704.9	2,726.5
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	246.1	159.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	7.1	42.3
Fixed maturities, including securities pledged, Fair Value	2,943.9	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	15,646.9	15,475.6
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	1,151.3	853.9
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	92.4	322.3
Fixed maturities, including securities pledged, Fair Value	$ 16,705.8	$ 16,007.2