Dividends (Dividends Paid) (Details) (USD $)	0 Months Ended	3 Months Ended												12 Months Ended
	Oct. 12, 2010	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010		Sep. 30, 2010		Jun. 30, 2010		Mar. 31, 2010		Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Dividends [Abstract]
Cash dividends per common share		$ 0.1600	$ 0.1588	$ 0.1575	$ 0.15625	$ 0.16	[1]	$ 0.15	[1]	$ 0.15	[1]	$ 0.14	[1]	$ 0.63	$ 0.6	$ 0.49
Total cash dividends paid		$ 10,734,000	$ 10,592,000	$ 10,500,000	$ 10,402,000									$ 42,228,000	$ 39,285,000	$ 32,246,000
Record date	Nov. 08, 2010	Oct. 28, 2011	Jul. 29, 2011	Apr. 22, 2011	Feb. 11, 2011									Feb. 24, 2012
Payment date	Nov. 12, 2010	Nov. 18, 2011	Aug. 19, 2011	May 13, 2011	Mar. 04, 2011									Mar. 16, 2012

[1] Year-to-date earnings and cash dividends per common share amounts may differ from the sum of quarterly amounts due to rounding.