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	$ 60,243
Reclassification adjustments for losses included in net income (interest expense)	127
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	464		464
Common stock redemptions	(719)		(719)
Share-based compensation	8,090	1	8,089
Net income	53,456				53,456
Reclassification adjustments for losses included in net income (interest expense)	369			369
Gains arising during the period on interest rate swaps	1,403			1,403
Dividends to common stockholders ($0.90 per share)	(112,694)					(112,694)
Ending balance at Sep. 30, 2018	$ 1,740,252	$ 1,252	$ 3,181,263	$ 473	$ 1,071,804	$ (2,514,540)