Investments - Composition of US and Foreign Corporate Securities (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	$ 1,222.6	$ 1,222.6
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	89.4	85.3
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	8.6	10.2
Fixed maturities, including securities pledged, Fair Value	1,303.4	1,297.7
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,750.2	2,850.4
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	149.7	146.7
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	6.9	14.1
Fixed maturities, including securities pledged, Fair Value	2,893.0	2,983.0
Industrial and other companies
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	8,318.2	8,479.5
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	386.2	346.4
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	62.7	98.9
Fixed maturities, including securities pledged, Fair Value	8,641.7	8,727.0
Energy
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,020.2	2,144.8
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	111.6	104.9
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	40.3	48.8
Fixed maturities, including securities pledged, Fair Value	2,091.5	2,200.9
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,486.4	2,436.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	138.0	130.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	25.9	31.3
Fixed maturities, including securities pledged, Fair Value	2,598.5	2,535.0
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	579.6	617.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	26.7	25.7
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	3.7	5.6
Fixed maturities, including securities pledged, Fair Value	602.6	637.8
U.S. and Foreign Corporate Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	17,377.2	17,751.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	901.6	839.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	148.1	208.9
Fixed maturities, including securities pledged, Fair Value	$ 18,130.7	$ 18,381.4