Fair Value of Financial Instruments - Fair Values of Corporates by Level and Sector (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Values of Corporates by Level and Sector:
Consumer	$ 677.8	$ 742.4
Energy	197.7	98.4
Financial services	672.7	527.7
Technical/communications	94.5	53.0
Transportation	84.3	39.1
Utilities	221.9	251.1
Other	174.2	185.8
Total corporates	2,123.1	1,897.5
Level 1
Fair Values of Corporates by Level and Sector:
Consumer	0	0
Energy	0	0
Financial services	0	0
Technical/communications	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	286.8	352.9
Energy	128.0	79.6
Financial services	434.3	361.9
Technical/communications	72.5	45.4
Transportation	20.3	9.3
Utilities	77.4	116.9
Other	88.6	130.0
Total corporates	1,107.9	1,096.0
Level 3
Fair Values of Corporates by Level and Sector:
Consumer	391.0	389.5
Energy	69.7	18.8
Financial services	238.4	165.8
Technical/communications	22.0	7.6
Transportation	64.0	29.8
Utilities	144.5	134.2
Other	85.6	55.8
Total corporates	$ 1,015.2	$ 801.5