Condensed Consolidated Statement of Equity (Unaudited) Statement - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Cumulative Net Income	Cumulative Dividends
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 57,070
Reclassification adjustments for losses included in net income (interest expense)	85
Beginning balance at Dec. 31, 2017	1,789,883	$ 1,251	$ 3,173,429	$ (1,299)	$ 1,018,348	$ (2,401,846)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock	351		351
Common stock redemptions	(680)		(680)
Share-based compensation	5,415	1	5,414
Net income	46,908				46,908
Reclassification adjustments for losses included in net income (interest expense)	274			274
Gains arising during the period on interest rate swaps	1,030			1,030
Dividends to common stockholders ($0.60 per share)	(75,125)					(75,125)
Ending balance at Jun. 30, 2018	$ 1,768,056	$ 1,252	$ 3,178,514	$ 5	$ 1,065,256	$ (2,476,971)