Stockholders' Equity (Stock Transactions - Narrative) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended			1 Months Ended	3 Months Ended	6 Months Ended			3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	0 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jul. 31, 2013 At The Market Equity Offering Program [Member]	Jun. 30, 2013 At The Market Equity Offering Program [Member] investment_bank	Jun. 30, 2013 At The Market Equity Offering Program [Member] investment_bank	Jun. 30, 2013 At The Market Equity Offering Program [Member] Minimum	Jun. 30, 2013 At The Market Equity Offering Program [Member] Maximum	Jun. 30, 2013 Restricted Stock	Jun. 30, 2013 Restricted Stock	Jun. 30, 2013 Employee Stock Purchase Plan	Jun. 30, 2013 Employee Stock Purchase Plan	Jul. 19, 2013 Subsequent Event	Apr. 30, 2013 Subsequent Event	Jul. 16, 2013 Subsequent Event
Class of Stock [Line Items]
Antidilutive securities excluded from computation of earnings per share (shares)											1,320,412	1,303,441	158,930	185,281
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									$ 24.19	$ 30.49							$ 26.13
Net proceeds from issuance of common stock			$ 132,416	$ 511		$ 9,100	$ 91,800	$ 131,400							$ 78,400
Shares sold under equity offering program						363,100	3,245,500	4,844,771
Equity Offering Program, Number of Investment Banks in Sales Agreements							4	4
Equity Offering Program, Shares Authorized to be Sold							9,000,000	9,000,000
Equity Offering Program, Shares Available for Issuance							5,391,400	5,391,400
Dividends per share paid to common stockholders (in dollars per share)			$ 0.6
Dividends per share declared to common stockholders (in dollars per share)	$ 0.30	$ 0.30	$ 0.60	$ 0.60												$ 0.30