Investments - Composition of US and Foreign Corporate Securities (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	$ 1,229.5	$ 1,218.8
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	103.3	67.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	10.9	28.6
Fixed maturities, including securities pledged, Fair Value	1,321.9	1,257.3
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,609.4	2,651.5
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	183.5	146.8
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	6.1	13.1
Fixed maturities, including securities pledged, Fair Value	2,786.8	2,785.2
Industrial and other companies
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	7,946.4	7,778.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	472.2	267.7
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	68.8	180.7
Fixed maturities, including securities pledged, Fair Value	8,349.8	7,865.2
Energy
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,408.4	2,655.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	52.4	26.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	181.5	261.8
Fixed maturities, including securities pledged, Fair Value	2,279.3	2,419.5
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,201.9	2,150.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	177.6	122.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	11.3	21.8
Fixed maturities, including securities pledged, Fair Value	2,368.2	2,251.0
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	569.5	560.6
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	32.2	14.0
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	2.8	13.8
Fixed maturities, including securities pledged, Fair Value	598.9	560.8
U.S. and Foreign Corporate Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	16,965.1	17,015.0
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	1,021.2	643.8
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	281.4	519.8
Fixed maturities, including securities pledged, Fair Value	$ 17,704.9	$ 17,139.0