Quarterly Financial Information (Unaudited) (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
Quarterly Financial Information
Revenues	$ 28,593	$ 25,825	$ 25,279	$ 25,277	$ 24,125	$ 23,402	$ 22,704	$ 22,251	$ 104,974		$ 92,482		$ 83,618
Net income (loss) from discontinued operations		2,857	(701)	254	254	253	251	263	2,410		1,021		823
Net income available to common stockholders	$ 13,732	$ 16,373	$ 11,994	$ 12,060	$ 11,854	$ 11,583	$ 12,194	$ 12,009	$ 54,159		$ 47,640		$ 39,832
Net income per common share from continuing operations available to common stockholders:
Basic (in dollars per share)	$ 0.40	$ 0.39	$ 0.39	$ 0.39	$ 0.38	$ 0.37	$ 0.40	$ 0.39	$ 1.56	[1]	$ 1.54	[1]	$ 1.34	[1]
Diluted (in dollars per share)	$ 0.40	$ 0.39	$ 0.39	$ 0.39	$ 0.38	$ 0.37	$ 0.39	$ 0.39	$ 1.56	[1]	$ 1.54	[1]	$ 1.33	[1]
Net income (loss) per common share from discontinued operations:
Basic (in dollars per share)	$ 0.00	$ 0.08	$ (0.02)	$ 0.01	$ 0.01	$ 0.01	$ 0.01	$ 0.01	$ 0.07	[1]	$ 0.03	[1]	$ 0.03	[1]
Diluted (in dollars per share)	$ 0.00	$ 0.08	$ (0.02)	$ 0.01	$ 0.01	$ 0.01	$ 0.01	$ 0.01	$ 0.07	[1]	$ 0.03	[1]	$ 0.02	[1]
Net income per common share available to common stockholders:
Basic (in dollars per share)	$ 0.40	$ 0.47	$ 0.36	$ 0.40	$ 0.39	$ 0.38	$ 0.40	$ 0.40	$ 1.64	[1]	$ 1.58	[1]	$ 1.36	[1]
Diluted (in dollars per share)	$ 0.40	$ 0.47	$ 0.36	$ 0.40	$ 0.39	$ 0.38	$ 0.40	$ 0.40	$ 1.63	[1]	$ 1.57	[1]	$ 1.36	[1]
Dividends per share declared (in dollars per share)	$ 0.510	$ 0.465	$ 0.465	$ 0.465	$ 0.465	$ 0.455	$ 0.435	$ 0.435
Dividend per share paid (in dollars per share)	$ 0.510	$ 0.465	$ 0.465	$ 0.465	$ 0.465	$ 0.455	$ 0.435	$ 0.435	$ 1.905		$ 1.79		$ 1.68

[1] NOTE: Computations of per share amounts from continuing operations, discontinued operations and net income are made independently. Therefore, the sum of per share amounts from continuing operations and discontinued operations may not agree with the per share amounts from net income available to common stockholders.