Dividends (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Dividends declared and paid
Dividend per Share	$ 0.24	$ 0.21	$ 0.20
Dividend 1 [Member]
Dividends declared and paid
Date Declared	Jan. 31, 2012	Jan. 21, 2011	Jan. 22, 2010
Record Date	Feb. 10, 2012	Feb. 01, 2011	Feb. 01, 2010
Payment Date	Feb. 15, 2012	Feb. 15, 2011	Feb. 15, 2010
Dividend per Share	$ 0.06	$ 0.05	$ 0.05
Dividend 2 [Member]
Dividends declared and paid
Date Declared	Apr. 19, 2012	Apr. 21, 2011	Apr. 20, 2010
Record Date	Apr. 30, 2012	May 02, 2011	May 03, 2010
Payment Date	May 15, 2012	May 13, 2011	May 14, 2010
Dividend per Share	$ 0.06	$ 0.05	$ 0.05
Dividend 3 [Member]
Dividends declared and paid
Date Declared	Jul. 19, 2012	Jul. 21, 2011	Jul. 19, 2010
Record Date	Jul. 30, 2012	Aug. 01, 2011	Jul. 29, 2010
Payment Date	Aug. 15, 2012	Aug. 15, 2011	Aug. 13, 2010
Dividend per Share	$ 0.06	$ 0.05	$ 0.05
Dividend 4 [Member]
Dividends declared and paid
Date Declared	Oct. 18, 2012	Oct. 25, 2011	Oct. 18, 2010
Record Date	Oct. 31, 2012	Nov. 04, 2011	Nov. 01, 2010
Payment Date	Nov. 15, 2012	Nov. 15, 2011	Nov. 15, 2010
Dividend per Share	$ 0.06	$ 0.06	$ 0.05