Stockholders' Equity (Stock Transactions - Narrative) (Details) $ / shares in Units, $ in Thousands			3 Months Ended		9 Months Ended
	Nov. 01, 2016 $ / shares	Jul. 05, 2016 USD ($) $ / shares shares	Sep. 30, 2016 $ / shares	Sep. 30, 2015 $ / shares	Sep. 30, 2016 USD ($) $ / shares shares	Sep. 30, 2015 USD ($) $ / shares	Oct. 28, 2016 shares	Mar. 31, 2016 investment_bank	Feb. 19, 2016 shares	Feb. 17, 2016 investment_bank	Dec. 31, 2015 $ / shares
Class of Stock [Line Items]
Issuance of common stock (shares) | shares		9,200,000
Net proceeds from issuance of common stock | $		$ 304,600			$ 450,304	$ 41,757
Dividends per share declared to common stockholders (in dollars per share)			$ 0.30	$ 0.30	$ 0.90	$ 0.90
Common stock, par value (dollars per share)		$ 0.01	$ 0.01		$ 0.01						$ 0.01
Share price (in dollars per share)		$ 33.13
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) | shares									10,000,000
Issuance of common stock (shares) | shares					4,795,601
Net proceeds from issuance of common stock | $					$ 144,600
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]
Dividends per share declared to common stockholders (in dollars per share)	$ 0.30
Subsequent Event [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Number of authorized shares remaining under offering program (shares) | shares							5,868,697
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