Stockholders' Equity (Stock Transactions - Narrative) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended	3 Months Ended		9 Months Ended				1 Months Ended	9 Months Ended			9 Months Ended		0 Months Ended
	Jul. 19, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Jul. 16, 2013	Dec. 31, 2012	Jul. 31, 2013 At The Market Equity Offering Program [Member]	Sep. 30, 2013 At The Market Equity Offering Program [Member] investment_bank	Sep. 30, 2013 At The Market Equity Offering Program [Member] Minimum	Sep. 30, 2013 At The Market Equity Offering Program [Member] Maximum	Sep. 30, 2013 Restricted Stock	Sep. 30, 2013 Employee Stock Purchase Plan	Oct. 29, 2013 Subsequent Event
Class of Stock [Line Items]
Antidilutive securities excluded from computation of earnings per share (shares)												1,294,953	164,418
Issuance of common stock (shares)	3,000,000
Common stock, par value		$ 0.01		$ 0.01		$ 0.01	$ 0.01
Sale of stock, price per share						$ 26.13				$ 24.19	$ 30.49
Net proceeds from issuance of common stock	$ 78,300			$ 220,045	$ 202,247			$ 9,100	$ 140,600
Shares sold under equity offering program								363,100	5,207,871
Equity Offering Program, Number of Investment Banks in Sales Agreements									4
Equity Offering Program, Shares Authorized to be Sold									9,000,000
Equity Offering Program, Shares Available for Issuance									5,391,400
Dividends per share paid to common stockholders (in dollars per share)				$ 0.90
Dividends per share declared to common stockholders (in dollars per share)		$ 0.30	$ 0.30	$ 0.90	$ 0.90									$ 0.30