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	$ 661.3	$ 742.4
Energy	172.5	98.4
Financial services	660.8	527.7
Technical/communications	99.1	53.0
Transportation	77.0	39.1
Utilities	215.5	251.1
Other	169.5	185.8
Total corporates	2,055.7	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	280.4	352.9
Energy	118.1	79.6
Financial services	438.9	361.9
Technical/communications	78.2	45.4
Transportation	18.1	9.3
Utilities	77.5	116.9
Other	88.6	130.0
Total corporates	1,099.8	1,096.0
Level 3
Fair Values of Corporates by Level and Sector:
Consumer	380.9	389.5
Energy	54.4	18.8
Financial services	221.9	165.8
Technical/communications	20.9	7.6
Transportation	58.9	29.8
Utilities	138.0	134.2
Other	80.9	55.8
Total corporates	$ 955.9	$ 801.5