Dividends (Dividend Payments) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Dividends [Abstract]
Cash dividends per common share (in dollars per share)	$ 0.17	[1]	$ 0.16875	[1]	$ 0.1675	[1]	$ 0.16625	[1]	$ 0.165	[1]	$ 0.16375	[1]	$ 0.1625	[1]	$ 0.16125	[1]
Total cash dividends paid	$ 11,947		$ 11,829		$ 11,516		$ 11,415										$ 46,707	$ 44,093	$ 42,228
Record date	Nov. 15, 2013		Aug. 16, 2013		May 10, 2013		Feb. 22, 2013
Payment date	Dec. 20, 2013		Sep. 20, 2013		Jun. 14, 2013		Mar. 15, 2013
Cash dividends per weighted average common share (in dollars per share)																	$ 0.67	$ 0.65	$ 0.63

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