Investments - Composition of US and Foreign Corporate Securities (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	$ 1,209.9	$ 1,218.8
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	133.3	67.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	5.1	28.6
Fixed maturities, including securities pledged, Fair Value	1,338.1	1,257.3
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,598.5	2,651.5
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	230.3	146.8
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	3.2	13.1
Fixed maturities, including securities pledged, Fair Value	2,825.6	2,785.2
Industrial and other companies
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	8,153.5	7,778.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	643.3	267.7
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	31.9	180.7
Fixed maturities, including securities pledged, Fair Value	8,764.9	7,865.2
Energy
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,177.1	2,655.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	118.0	26.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	83.7	261.8
Fixed maturities, including securities pledged, Fair Value	2,211.4	2,419.5
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,232.1	2,150.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	248.0	122.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	4.8	21.8
Fixed maturities, including securities pledged, Fair Value	2,475.3	2,251.0
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	570.5	560.6
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	51.5	14.0
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	1.8	13.8
Fixed maturities, including securities pledged, Fair Value	620.2	560.8
U.S. and Foreign Corporate Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	16,941.6	17,015.0
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	1,424.4	643.8
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	130.5	519.8
Fixed maturities, including securities pledged, Fair Value	$ 18,235.5	$ 17,139.0