Investments - Composition of US and Foreign Corporate Securities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Communications
Available-for-sale Securities, Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	$ 4,009.4		$ 4,016.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	503.4		293.0
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	11.5		73.4
Fixed maturities, including securities pledged	4,501.3		4,235.8
Financial
Available-for-sale Securities, Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	7,205.7		6,640.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	753.8		566.6
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	11.3		72.6
Fixed maturities, including securities pledged	7,948.2		7,134.7
Industrial and Other Companies
Available-for-sale Securities, Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	29,995.9		29,303.1
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	2,346.1		1,524.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	151.5		564.5
Fixed maturities, including securities pledged	32,190.5		30,263.1
Utilities
Available-for-sale Securities, Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	9,212.7		9,200.6
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	954.6		570.0
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	29.4		142.2
Fixed maturities, including securities pledged	10,137.9		9,628.4
Transportation
Available-for-sale Securities, Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	1,405.3		1,467.1
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	141.0		84.6
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	5.9		30.0
Fixed maturities, including securities pledged	1,540.4		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	51,829.0		50,627.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	4,698.9		3,038.7
U.S. and foreign corporate securities, Gross Unrealized Capital Losses		209.6	882.7
Fixed maturities, including securities pledged	$ 56,318.3		$ 52,783.7