Subsequent Events (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Subsequent Event [Line Items]
Subsequent Event, Date			Aug. 01, 2013
Subsequent Event, Description			two-for-one stock split of our outstanding shares of common stock, to be effected in the form of a stock dividend, payable on September 26, 2013 to shareholders of record on September 12, 2013
Net income per share, basic	$ 0.87	$ 0.46	$ 1.79	$ 1.32
Net income per share, diluted	$ 0.86	$ 0.45	$ 1.76	$ 1.27
Dividends declared per share	$ 0.19	$ 0.18	$ 0.38	$ 0.36
Pro Forma [Member] | Two-for-One Stock Split [Member]
Subsequent Event [Line Items]
Net income per share, basic	$ 0.435	$ 0.230	$ 0.895	$ 0.660
Net income per share, diluted	$ 0.430	$ 0.225	$ 0.880	$ 0.635
Dividends declared per share	$ 0.095	$ 0.090	$ 0.190	$ 0.180