Stockholders' Equity (Issuance Of Common Stock Through "At-The-Market" Equity Offering Program) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended						9 Months Ended	12 Months Ended
	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2009	Jun. 30, 2010	Dec. 31, 2011
Stockholders' Equity [Abstract]
Number of Shares Sold	877,400	1,485,707	841,000	1,021,400	1,285,700	871,600	3,178,700	3,204,107
Gross Proceeds	$ 56,542	$ 90,102	$ 47,524	$ 50,078	$ 60,092	$ 39,830	$ 150,000	$ 194,168
Net Proceeds	$ 55,273	$ 88,299	$ 46,572	$ 49,273	$ 58,856	$ 38,916	$ 147,045	$ 190,144
Average Sales Price	$ 64.44	$ 60.65	$ 56.51	$ 49.03	$ 46.74	$ 45.70	$ 47.19	$ 60.60