Investments - Composition of US and Foreign Corporate Securities (Details) (USD $) In Millions, unless otherwise specified	Sep. 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	$ 3,959.6		$ 4,016.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	455.8		293.0
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	15.5		73.4
Fixed maturities, including securities pledged	4,399.9		4,235.8
Financial
Available-for-sale Securities, Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	7,582.8		6,640.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	669.3		566.6
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	20.1		72.6
Fixed maturities, including securities pledged	8,232.0		7,134.7
Industrial and Other Companies
Available-for-sale Securities, Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	30,002.1		29,303.1
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	1,982.7		1,524.5
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	204.4		564.5
Fixed maturities, including securities pledged	31,780.4		30,263.1
Utilities
Available-for-sale Securities, Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	9,335.1		9,200.6
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	884.0		570.0
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	30.0		142.2
Fixed maturities, including securities pledged	10,189.1		9,628.4
Transportation
Available-for-sale Securities, Including Securities Pledged [Line Items]
U.S. and foreign securities, Amortized Cost	1,478.9		1,467.1
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	127.8		84.6
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	9.4		30.0
Fixed maturities, including securities pledged	1,597.3		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	52,358.5		50,627.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	4,119.6		3,038.7
U.S. and foreign corporate securities, Gross Unrealized Capital Losses		279.4	882.7
Fixed maturities, including securities pledged	$ 56,198.7		$ 52,783.7