Investments - Composition of US and Foreign Corporate Securities (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	$ 1,186.8	$ 1,226.1
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	81.9	136.8
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	15.4	2.4
Fixed maturities, including securities pledged, Fair Value	1,253.3	1,360.5
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	2,395.4	2,310.5
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	164.0	221.4
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	11.4	1.6
Fixed maturities, including securities pledged, Fair Value	2,548.0	2,530.3
Industrial and other companies
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	9,580.0	8,962.6
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	401.7	569.4
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	167.3	90.0
Fixed maturities, including securities pledged, Fair Value	9,814.4	9,442.0
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	2,437.1	2,555.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	166.7	259.2
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	16.2	4.3
Fixed maturities, including securities pledged, Fair Value	2,587.6	2,810.6
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	484.4	450.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	24.6	40.3
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	5.4	1.3
Fixed maturities, including securities pledged, Fair Value	503.6	489.7
U.S. and Foreign Corporate Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	16,083.7	15,505.6
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	838.9	1,227.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	215.7	99.6
Fixed maturities, including securities pledged, Fair Value	$ 16,706.9	$ 16,633.1