Debt (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Mar. 31, 2012
Debt Instrument [Line Items]
Carrying Value	$ 2,063.5	[1]	$ 2,063.1	[1]	$ 2,080.0	[1]
Fair Value	2,397.2		2,394.4		2,337.0
7% Notes due 2013
Debt Instrument [Line Items]
Carrying Value	149.9		149.9		149.8
Fair Value	154.9		157.1		163.1
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Carrying Value	497.4		497.3		497.1
Fair Value	544.7		549.1		532.2
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Carrying Value	295.2		295.2		295.0
Fair Value	382.6		385.0		370.2
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Carrying Value	394.5		394.5		394.4
Fair Value	507.1		513.5		486.4
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Carrying Value	726.5		726.2		743.7
Fair Value	$ 807.9		$ 789.7		$ 785.1

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