Summary Of Common Stock Sales Price And Dividends Declared Per Share (Detail) (USD $)	3 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011
Dividends Payable [Line Items]
Dividend Declared	$ 0.70	$ 0.80	$ 0.90	$ 0.90	$ 0.90	$ 1.00	$ 1.00	$ 1.00
Maximum
Dividends Payable [Line Items]
Sales Price	$ 28.15	$ 29.68	$ 29.54	$ 29.00	$ 27.31	$ 29.26	$ 29.39	$ 30.99
Minimum
Dividends Payable [Line Items]
Sales Price	$ 24.40	$ 27.87	$ 27.88	$ 26.24	$ 22.33	$ 23.80	$ 27.12	$ 27.88