Debt (Detail) (USD $)	Jun. 30, 2014		Dec. 31, 2013		Jun. 30, 2013
Debt Instrument [Line Items]
Carrying Value	$ 2,208,000,000	[1]	$ 1,860,900,000	[1]	$ 2,063,900,000	[1]
Fair Value	2,551,600,000		2,073,700,000		2,300,800,000
7% Notes due 2013
Debt Instrument [Line Items]
Carrying Value	0		0		150,000,000
Fair Value	0		0		152,300,000
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Carrying Value	497,700,000		497,600,000		497,400,000
Fair Value	532,800,000		509,100,000		517,600,000
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Carrying Value	295,400,000		295,300,000		295,200,000
Fair Value	395,900,000		359,600,000		362,100,000
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Carrying Value	394,700,000		394,600,000		394,600,000
Fair Value	514,700,000		473,700,000		479,100,000
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Carrying Value	346,300,000		0		0
Fair Value	355,000,000		0		0
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Carrying Value	673,900,000		673,400,000		726,700,000
Fair Value	$ 753,200,000		$ 731,300,000		$ 789,700,000

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