Stockholders' Equity (Stock Transactions - Narrative) (Details) $ / shares in Units, $ in Thousands			3 Months Ended		6 Months Ended
	Aug. 02, 2016 $ / shares	Jul. 05, 2016 USD ($) $ / shares shares	Jun. 30, 2016 $ / shares shares	Jun. 30, 2015 $ / shares	Jun. 30, 2016 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) $ / shares	Mar. 31, 2016 investment_bank	Feb. 19, 2016 shares	Feb. 17, 2016 investment_bank	Dec. 31, 2015 $ / shares
Class of Stock [Line Items]
Net proceeds from issuance of common stock | $					$ 145,125	$ 40,366
Dividends per share declared to common stockholders (in dollars per share)			$ 0.30	$ 0.30	$ 0.6	$ 0.60
Common stock, par value (dollars per share)			$ 0.01		$ 0.01					$ 0.01
At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Number of investment banks in sales agreements | investment_bank							5
Shares authorized to be sold under the program | shares								10,000,000
Issuance of common stock (shares) | shares					4,795,601
Net proceeds from issuance of common stock | $					$ 144,600
Number of authorized shares remaining under offering program | shares			5,868,697		5,868,697
Previous At the Market Equity Offering Program [Member]
Class of Stock [Line Items]
Number of investment banks in sales agreements | investment_bank									1
Issuance of common stock (shares) | shares					664,298
Subsequent Event [Member]
Class of Stock [Line Items]
Issuance of common stock (shares) | shares		9,200,000
Net proceeds from issuance of common stock | $		$ 304,600
Dividends per share declared to common stockholders (in dollars per share)	$ 0.30
Common stock, par value (dollars per share)		$ 0.01
Share price		$ 33.13
Minimum [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Issuance of common stock (price per share)					$ 28.31
Maximum [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Issuance of common stock (price per share)					33.66
Weighted Average [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Issuance of common stock (price per share)					$ 30.61
Unsecured Credit Facility Due 2017 [Member] | Line of Credit [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Repayment of principal | $		$ 190,000
Repayment of loan | $		190,000
Term Loan Due 2019 [Member] | Medium-term Notes [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Repayment of loan | $		$ 50,000