Quarterly Financial Data (Unaudited) (Details 2) (USD $)	3 Months Ended								12 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	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Summary of range high and low closing sales prices and dividends declared
Cash dividends	$ 0.12	$ 0.09	$ 0.09	$ 0.07	$ 0.07	$ 0.05	$ 0.05	$ 0.05	$ 0.37	$ 0.22	$ 0.20	$ 0.20	$ 0.80
Maximum [Member]
Summary of range high and low closing sales prices and dividends declared
Closing price	$ 16.75	$ 15.30	$ 14.50	$ 12.65	$ 12.55	$ 12.69	$ 12.85	$ 13.25	$ 16.75	$ 12.55
Minimum [Member]
Summary of range high and low closing sales prices and dividends declared
Closing price	$ 13.50	$ 13.25	$ 12.50	$ 11.20	$ 11.05	$ 11.80	$ 11.90	$ 12.00	$ 13.50	$ 11.05