Shareholders' Equity - Reconciliation of Changes in Parent Company's Total Shareholders' Equity (Detail) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Beginning Balance	$ 2,132,681	$ 2,147,719	$ 2,132,894	$ 2,041,920	$ 2,046,891	$ 2,057,737	$ 2,132,894	$ 2,057,737
Issuance of non-vested stock					1
Exercise of stock options				469
Net proceeds from issuance of common stock	134,226		21,466
Earned portion of non-vested stock	1,107	1,071	1,124	955	839	1,396
Adjustment to redemption value on noncontrolling redeemable Operating Partnership units	(2,948)	(2,609)	5,542	(1,606)	(150)	(294)	(15)
Purchases of equity in consolidated subsidiary from noncontrolling interests	(2,300)
Net income attributable to common shareholders	37,095	36,457	36,433	140,002	40,742	34,454	109,984	215,198
Amortization of terminated hedge included in AOCL	230	229	229	229	229	229
Dividends	(50,203)	(50,186)	(49,969)	(46,651)	(46,632)	(46,631)
Ending Balance	2,249,888	2,132,681	2,147,719	2,135,318	2,041,920	2,046,891	2,249,888	2,135,318
Common Stock Shares [Member]
Beginning Balance	469	469	467	467	466	466	467	466
Issuance of non-vested stock					1
Net proceeds from issuance of common stock	13		2
Ending Balance	482	469	469	467	467	466	482	467
Additional Paid-in Capital [Member]
Beginning Balance	2,400,382	2,399,311	2,376,723	2,374,392	2,373,553	2,372,157	2,376,723	2,372,157
Exercise of stock options				469
Net proceeds from issuance of common stock	134,213		21,464
Earned portion of non-vested stock	1,107	1,071	1,124	955	839	1,396
Purchases of equity in consolidated subsidiary from noncontrolling interests	(2,300)
Ending Balance	2,533,402	2,400,382	2,399,311	2,375,816	2,374,392	2,373,553	2,533,402	2,375,816
Dividends in Excess of Net Income [Member]
Beginning Balance	(262,670)	(246,332)	(238,338)	(326,522)	(320,482)	(308,011)	(238,338)	(308,011)
Adjustment to redemption value on noncontrolling redeemable Operating Partnership units	(2,948)	(2,609)	5,542	(1,606)	(150)	(294)
Net income attributable to common shareholders	37,095	36,457	36,433	140,002	40,742	34,454
Dividends	(50,203)	(50,186)	(49,969)	(46,651)	(46,632)	(46,631)
Ending Balance	(278,726)	(262,670)	(246,332)	(234,777)	(326,522)	(320,482)	(278,726)	(234,777)
Accumulated Other Comprehensive Income (Loss) [Member]
Beginning Balance	(5,500)	(5,729)	(5,958)	(6,417)	(6,646)	(6,875)	(5,958)	(6,875)
Amortization of terminated hedge included in AOCL	230	229	229	229	229	229
Ending Balance	$ (5,270)	$ (5,500)	$ (5,729)	$ (6,188)	$ (6,417)	$ (6,646)	$ (5,270)	$ (6,188)