Debt - Additional Information (Detail) (USD $)	1 Months Ended
	Jan. 31, 2012	Sep. 30, 2012 FinancialCovenants	Sep. 30, 2012 5.95% Notes and Debentures due March 15, 2022 [Member]	Mar. 05, 2012 5.95% Notes and Debentures due March 15, 2022 [Member]	Sep. 30, 2012 5.875% Notes and Debentures due July 16, 2012 [Member]	Jul. 16, 2012 5.875% Notes and Debentures due July 16, 2012 [Member]
Debt Instrument [Line Items]
Interest on notes			5.95%		5.875%
Issued notes		$ 400,000,000		$ 400,000,000		$ 745,000,000
Effective interest rate for notes		6.50%
Company repurchased Notes in open-market transactions	46,000,000
Premium paid on repurchase of notes	1,000,000
Additional borrowing capacity		1,000,000,000
Absorption of reduction to shareholders' equity to remain in compliance with covenant		547,000,000
Financial covenants		2
Borrowings under Five Year Revolving Credit Agreement		$ 0