Fair Value of Financial Instruments - Fair Values of Corporates by Level and Sector (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Values of Corporates by Level and Sector:
Consumer	$ 550.3	$ 665.9
Energy	298.9	233.1
Financial services	898.0	702.5
Capital goods	263.1	119.7
Transportation	138.7	110.1
Utilities	279.1	237.9
Other	270.1	138.1
Total corporates	2,698.2	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	225.4	288.9
Energy	199.0	137.7
Financial services	605.9	440.3
Capital goods	153.4	62.6
Transportation	32.1	25.0
Utilities	104.5	83.6
Other	150.1	66.9
Total corporates	1,470.4	1,105.0
Level 3
Fair Values of Corporates by Level and Sector:
Consumer	324.9	377.0
Energy	99.9	95.4
Financial services	292.1	262.2
Capital goods	109.7	57.1
Transportation	106.6	85.1
Utilities	174.6	154.3
Other	120.0	71.2
Total corporates	$ 1,227.8	$ 1,102.3