Dividends (Quarterly Dividend Payments) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Dividends [Abstract]
Cash dividends per common share (in dollars per share)	$ 0.175	[1]	$ 0.17375	[1]	$ 0.17250	[1]	$ 0.17125	[1]	$ 0.17	[1]	$ 0.16875	[1]	$ 0.1675	[1]	$ 0.16625	[1]
Total cash dividends paid	$ 12,479		$ 12,335		$ 12,186		$ 12,077										$ 49,077	$ 46,707	$ 44,093
Record date	Nov. 21, 2014		Aug. 22, 2014		May 23, 2014		Feb. 21, 2014
Payment date	Dec. 26, 2014		Sep. 26, 2014		Jun. 27, 2014		Mar. 28, 2014

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