Fair Value of Financial Instruments - Fair Values of Corporates by Level and Sector (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Fair Values of Corporates by Level and Sector:
Consumer	$ 513.2	$ 665.9
Energy	249.4	233.1
Financial services	738.0	702.5
Capital goods	233.4	119.7
Transportation	125.4	110.1
Utilities	249.5	237.9
Other	240.0	138.1
Total corporates	2,348.9	2,207.3
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	206.0	288.9
Energy	155.3	137.7
Financial services	489.9	440.3
Capital goods	133.2	62.6
Transportation	29.7	25.0
Utilities	92.0	83.6
Other	120.5	66.9
Total corporates	1,226.6	1,105.0
Level 3
Fair Values of Corporates by Level and Sector:
Consumer	307.2	377.0
Energy	94.1	95.4
Financial services	248.1	262.2
Capital goods	100.2	57.1
Transportation	95.7	85.1
Utilities	157.5	154.3
Other	119.5	71.2
Total corporates	$ 1,122.3	$ 1,102.3