Dividends (Dividends Paid) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	0 Months Ended	3 Months Ended																12 Months Ended
	Oct. 12, 2010	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
Dividends [Abstract]
Cash dividends per common share (dollars per share)		$ 0.16500	[1]	$ 0.16375	[1]	$ 0.16250	[1]	$ 0.16125	[1]	$ 0.16	[1]	$ 0.16	[1]	$ 0.16	[1]	$ 0.16	[1]	$ 0.65	$ 0.63	$ 0.60
Total cash dividends paid		$ 11,227		$ 11,063		$ 10,956		$ 10,847										$ 44,093	$ 42,228	$ 39,285
Record date	Nov. 08, 2010	Oct. 26, 2012		Jul. 27, 2012		Apr. 27, 2012		Feb. 24, 2012
Payment date	Nov. 12, 2010	Nov. 30, 2012		Aug. 24, 2012		May 18, 2012		Mar. 26, 2012

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