Equity - Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended							9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Equity activity
Balance at beginning of period	$ 805,696	$ 756,863	$ 758,648	$ 756,635	$ 757,167	$ 753,140	$ 740,048	$ 758,648	$ 740,048
Common Stock cash distributions	(22,600)	(22,590)	(22,578)	(22,555)	(22,554)	(22,558)	(22,552)
Proceeds from common stock issued, net of issuance costs	929						14,529
Stock-based compensation expense	1,487	1,521	1,376	1,282	1,283	1,425	1,259
Net income	34,937	68,936	20,359	19,834	20,616	25,377	21,513	124,232	67,506
Non-controlling interests contribution	2,882	1,081		3,488
Non-controlling interest distributions	(17)
Other	(21)	(107)	(1,065)	(22)		(296)	(1,651)
Balance at end of period	823,293	805,696	756,863	758,648	756,635	757,167	753,140	823,293	756,635
Parent
Equity activity
Balance at beginning of period	801,127	753,375	755,160	756,635	757,167	753,140	740,048	755,160	740,048
Common Stock cash distributions	(22,600)	(22,590)	(22,578)	(22,555)	(22,554)	(22,558)	(22,552)
Proceeds from common stock issued, net of issuance costs	929						14,529
Stock-based compensation expense	1,487	1,521	1,376	1,282	1,283	1,425	1,259
Net income	34,920	68,936	20,359	19,834	20,616	25,377	21,513
Other	(21)	(107)	(1,065)	(22)		(296)	(1,651)
Balance at end of period	815,842	801,127	753,375	755,160	756,635	757,167	753,140	815,842	756,635
Common Stock
Equity activity
Balance at beginning of period	396	396	396	396	396	396	392	396	392
Proceeds from common stock issued, net of issuance costs	1						3
Balance at end of period	397	396	396	396	396	396	396	397	396
Capital in Excess of Par Value
Equity activity
Balance at beginning of period	858,832	857,426	856,992	855,746	854,340	853,132	839,005	856,992	839,005
Proceeds from common stock issued, net of issuance costs	928						14,526
Stock-based compensation expense	1,487	1,521	1,376	1,282	1,283	1,425	1,259
Other	(21)	(107)	(1,065)	(22)		(296)	(1,651)
Balance at end of period	861,226	858,832	857,426	856,992	855,746	854,340	853,132	861,226	855,746
Cumulative Net Income
Equity activity
Balance at beginning of period	1,190,078	1,121,142	1,100,783	1,080,949	1,060,333	1,034,956	1,013,443	1,100,783	1,013,443
Net income	34,920	68,936	20,359	19,834	20,616	25,377	21,513
Balance at end of period	1,224,998	1,190,078	1,121,142	1,100,783	1,080,949	1,060,333	1,034,956	1,224,998	1,080,949
Cumulative Distributions
Equity activity
Balance at beginning of period	(1,248,179)	(1,225,589)	(1,203,011)	(1,180,456)	(1,157,902)	(1,135,344)	(1,112,792)	(1,203,011)	(1,112,792)
Common Stock cash distributions	(22,600)	(22,590)	(22,578)	(22,555)	(22,554)	(22,558)	(22,552)
Balance at end of period	(1,270,779)	(1,248,179)	(1,225,589)	(1,203,011)	$ (1,180,456)	$ (1,157,902)	$ (1,135,344)	(1,270,779)	$ (1,180,456)
Non-controlling Interests
Equity activity
Balance at beginning of period	4,569	3,488	3,488					3,488
Net income	17
Non-controlling interests contribution	2,882	1,081		3,488
Non-controlling interest distributions	(17)
Balance at end of period	$ 7,451	$ 4,569	$ 3,488	$ 3,488				$ 7,451