Debt (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012		Sep. 30, 2012
Debt Instrument [Line Items]
Carrying Value	$ 2,010.6	[1]	$ 2,063.1	[1]	$ 2,062.7	[1]
Fair Value	2,231.4		2,394.4		2,405.9
7% Notes due 2013
Debt Instrument [Line Items]
Carrying Value	150.0		149.9		149.8
Fair Value	150.0		157.1		159.0
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Carrying Value	497.5		497.3		497.3
Fair Value	516.7		549.1		551.0
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Carrying Value	295.3		295.2		295.1
Fair Value	362.0		385.0		393.7
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Carrying Value	394.6		394.5		394.5
Fair Value	478.2		513.5		521.7
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Carrying Value	673.2		726.2		726.0
Fair Value	$ 724.5		$ 789.7		$ 780.5

[1] Consists of both short- and long-term debt. See Note 4 - Debt.