Investments - Composition of US and Foreign Corporate Securities (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	$ 1,216.6	$ 1,218.8
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	143.1	67.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	1.0	28.6
Fixed maturities, including securities pledged, Fair Value	1,358.7	1,257.3
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,740.5	2,651.5
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	243.1	146.8
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	2.6	13.1
Fixed maturities, including securities pledged, Fair Value	2,981.0	2,785.2
Industrial and other companies
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	8,331.7	7,778.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	674.6	267.7
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	17.4	180.7
Fixed maturities, including securities pledged, Fair Value	8,988.9	7,865.2
Energy
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,185.4	2,655.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	146.0	26.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	61.9	261.8
Fixed maturities, including securities pledged, Fair Value	2,269.5	2,419.5
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,328.6	2,150.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	243.7	122.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	5.3	21.8
Fixed maturities, including securities pledged, Fair Value	2,567.0	2,251.0
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	602.3	560.6
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	53.1	14.0
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	0.7	13.8
Fixed maturities, including securities pledged, Fair Value	654.7	560.8
U.S. and Foreign Corporate Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	17,405.1	17,015.0
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	1,503.6	643.8
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	88.9	519.8
Fixed maturities, including securities pledged, Fair Value	$ 18,819.8	$ 17,139.0