Investments - Composition of US and Foreign Corporate Securities (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	$ 1,192.7	$ 1,222.6
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	112.0	85.3
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	2.0	10.2
Fixed maturities, including securities pledged, Fair Value	1,302.7	1,297.7
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,754.4	2,850.4
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	178.3	146.7
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	3.5	14.1
Fixed maturities, including securities pledged, Fair Value	2,929.2	2,983.0
Industrial and other companies
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	8,195.4	8,479.5
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	484.9	346.4
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	34.3	98.9
Fixed maturities, including securities pledged, Fair Value	8,646.0	8,727.0
Energy
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,984.1	2,144.8
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	132.0	104.9
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	43.8	48.8
Fixed maturities, including securities pledged, Fair Value	2,072.3	2,200.9
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,571.3	2,436.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	179.1	130.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	14.9	31.3
Fixed maturities, including securities pledged, Fair Value	2,735.5	2,535.0
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	586.5	617.7
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	36.5	25.7
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	1.7	5.6
Fixed maturities, including securities pledged, Fair Value	621.3	637.8
U.S. and Foreign Corporate Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	17,284.4	17,751.2
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	1,122.8	839.1
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	100.2	208.9
Fixed maturities, including securities pledged, Fair Value	$ 18,307.0	$ 18,381.4