Investments - Composition of US and Foreign Corporate Securities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	$ 1,169.9	$ 1,222.6
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	111.4	85.3
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	2.9	10.2
Fixed maturities, including securities pledged, Fair Value	1,278.4	1,297.7
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,794.5	2,850.4
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	188.4	146.7
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	2.5	14.1
Fixed maturities, including securities pledged, Fair Value	2,980.4	2,983.0
Industrial and other companies
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	8,234.7	8,479.5
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	525.5	346.4
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	28.9	98.9
Fixed maturities, including securities pledged, Fair Value	8,731.3	8,727.0
Energy
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,008.0	2,144.8
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	142.7	104.9
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	36.2	48.8
Fixed maturities, including securities pledged, Fair Value	2,114.5	2,200.9
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,656.9	2,436.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	194.3	130.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	13.0	31.3
Fixed maturities, including securities pledged, Fair Value	2,838.2	2,535.0
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	688.6	617.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	47.1	25.7
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	1.4	5.6
Fixed maturities, including securities pledged, Fair Value	734.3	637.8
U.S. and Foreign Corporate Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	17,552.6	17,751.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	1,209.4	839.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	84.9	208.9
Fixed maturities, including securities pledged, Fair Value	$ 18,677.1	$ 18,381.4