Fair Value of Financial Instruments - Fair Values of Corporates by Level and Sector (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fair Values of Corporates by Level and Sector:
Consumer	$ 609.5	$ 582.5
Energy	384.4	344.7
Financial services	976.5	896.1
Capital goods	261.3	267.3
Transportation	158.8	145.4
Utilities	380.8	337.2
Other	304.4	258.7
Total corporates	3,075.7	2,831.9
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	227.5	219.9
Energy	223.7	181.6
Financial services	667.5	574.6
Capital goods	142.6	142.0
Transportation	38.2	30.5
Utilities	137.6	117.9
Other	186.1	148.2
Total corporates	1,623.2	1,414.7
Level 3
Fair Values of Corporates by Level and Sector:
Consumer	382.0	362.6
Energy	160.7	163.1
Financial services	309.0	321.5
Capital goods	118.7	125.3
Transportation	120.6	114.9
Utilities	243.2	219.3
Other	118.3	110.5
Total corporates	$ 1,452.5	$ 1,417.2