Fair Value, Assets Measured on Recurring Basis (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Values of Corporates by Level and Sector:
Consumer	$ 582.5	$ 656.8
Energy	344.7	273.0
Financial services	896.1	678.3
Capital goods	267.3	118.2
Transportation	145.4	110.1
Utilities	337.2	237.9
Other	258.7	122.6
Total corporates	2,831.9	2,196.9
Level 1
Fair Values of Corporates by Level and Sector:
Consumer	0	0
Energy	0	0
Financial services	0	0
Capital goods	0	0
Transportation	0	0
Utilities	0	0
Other	0	0
Total corporates	0	0
Level 2
Fair Values of Corporates by Level and Sector:
Consumer	219.9	288.9
Energy	181.6	137.7
Financial services	574.6	440.3
Capital goods	142.0	62.6
Transportation	30.5	25.0
Utilities	117.9	83.6
Other	148.2	66.9
Total corporates	1,414.7	1,105.0
Level 3
Fair Values of Corporates by Level and Sector:
Consumer	362.6	367.9
Energy	163.1	135.3
Financial services	321.5	238.0
Capital goods	125.3	55.6
Transportation	114.9	85.1
Utilities	219.3	154.3
Other	110.5	55.7
Total corporates	$ 1,417.2	$ 1,091.9