Stockholders' Equity (Stock Transactions - Narrative) (Details) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
	Aug. 04, 2015 $ / shares	Jun. 30, 2015 USD ($) investment_bank $ / shares shares	Jun. 30, 2014 $ / shares	Jun. 30, 2015 USD ($) investment_bank $ / shares shares	Jun. 30, 2014 USD ($) $ / shares
Class of Stock [Line Items]
Net proceeds from issuance of common stock | $				$ 40,366	$ 27,873
Dividends per share paid to common stockholders (in dollars per share)				$ 0.60
Dividends per share declared to common stockholders (in dollars per share)		$ 0.30	$ 0.30	$ 0.60	$ 0.60
At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Issuance of common stock (shares) | shares		304,752		1,445,114
Net proceeds from issuance of common stock | $		$ 8,300		$ 39,500
Number of investment banks in sales agreements | investment_bank		4		4
Shares authorized to be sold under the program | shares		9,000,000		9,000,000
Number of authorized shares remaining under offering program | shares		936,525		936,525
Subsequent Event [Member]
Class of Stock [Line Items]
Dividends per share declared to common stockholders (in dollars per share)	$ 0.30
Minimum [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Issuance of common stock (price per share)		$ 27.04		$ 26.35
Maximum [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Issuance of common stock (price per share)		28.00		29.15
Weighted Average [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Issuance of common stock (price per share)		$ 27.77		$ 27.78