Fair Value of Financial Instruments - Fair Values of Corporates by Level and Sector (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Fair Values of Corporates by Level and Sector:
Consumer	$ 532.8	$ 665.9
Energy	265.0	233.1
Financial services	807.2	702.5
Capital goods	257.3	119.7
Transportation	128.9	110.1
Utilities	259.6	237.9
Other	252.4	138.1
Total corporates	2,503.2	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	216.8	288.9
Energy	166.2	137.7
Financial services	558.1	440.3
Capital goods	154.1	62.6
Transportation	29.9	25.0
Utilities	99.6	83.6
Other	133.4	66.9
Total corporates	1,358.1	1,105.0
Level 3
Fair Values of Corporates by Level and Sector:
Consumer	316.0	377.0
Energy	98.8	95.4
Financial services	249.1	262.2
Capital goods	103.2	57.1
Transportation	99.0	85.1
Utilities	160.0	154.3
Other	119.0	71.2
Total corporates	$ 1,145.1	$ 1,102.3