Fair Value of Financial Instruments - Fair Values of Corporates by Level and Sector (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Values of Corporates by Level and Sector:
Consumer	$ 649.4	$ 742.4
Energy	174.5	98.4
Financial services	654.2	527.7
Technical/communications	98.0	53.0
Transportation	75.3	39.1
Utilities	202.5	251.1
Other	171.6	185.8
Total corporates	2,025.5	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	302.5	352.9
Energy	116.5	79.6
Financial services	436.3	361.9
Technical/communications	73.9	45.4
Transportation	15.5	9.3
Utilities	79.1	116.9
Other	98.5	130.0
Total corporates	1,122.3	1,096.0
Level 3
Fair Values of Corporates by Level and Sector:
Consumer	346.9	389.5
Energy	58.0	18.8
Financial services	217.9	165.8
Technical/communications	24.1	7.6
Transportation	59.8	29.8
Utilities	123.4	134.2
Other	73.1	55.8
Total corporates	$ 903.2	$ 801.5