Shareholders' Equity - Reconciliation of Changes in Parent Company's Total Shareholders' Equity (Detail) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Beginning Balance	$ 2,046,891	$ 2,057,737	$ 2,019,337	$ 2,028,323	$ 2,057,737	$ 2,028,323
Issuance of non-vested stock	1
Exercise of stock options			3,045
Earned portion of non-vested stock	839	1,396	1,252	1,495
Stock option expense			3	4
Carrying value less than redemption value on redeemed noncontrolling interest			(28)
Adjustment to redemption value on noncontrolling redeemable Operating Partnership units	(150)	(294)	(3,035)	1,470	(444)
Net income attributable to common shareholders	40,742	34,454	39,274	33,889	75,196	73,163
Amortization of terminated hedge included in AOCL	229	229	229	229
Change in fair value of derivatives, net of reclassifications			(123)	48
Dividends	(46,632)	(46,631)	(46,514)	(46,121)
Ending Balance	2,041,920	2,046,891	2,013,440	2,019,337	2,041,920	2,013,440
Common Stock Shares [Member]
Beginning Balance	466	466	465	466	466	466
Issuance of non-vested stock	1
Forfeiture of non-vested stock				(1)
Exercise of stock options			1
Ending Balance	467	466	466	465	467	466
Additional Paid-in Capital [Member]
Beginning Balance	2,373,553	2,372,157	2,364,671	2,363,171	2,372,157	2,363,171
Forfeiture of non-vested stock				1
Exercise of stock options			3,044
Earned portion of non-vested stock	839	1,396	1,252	1,495
Stock option expense			3	4
Carrying value less than redemption value on redeemed noncontrolling interest			(28)
Ending Balance	2,374,392	2,373,553	2,368,942	2,364,671	2,374,392	2,368,942
Dividends in Excess of Net Income [Member]
Beginning Balance	(320,482)	(308,011)	(338,489)	(327,727)	(308,011)	(327,727)
Adjustment to redemption value on noncontrolling redeemable Operating Partnership units	(150)	(294)	(3,035)	1,470
Net income attributable to common shareholders	40,742	34,454	39,274	33,889
Dividends	(46,632)	(46,631)	(46,514)	(46,121)
Ending Balance	(326,522)	(320,482)	(348,764)	(338,489)	(326,522)	(348,764)
Accumulated Other Comprehensive Income (Loss) [Member]
Beginning Balance	(6,646)	(6,875)	(7,310)	(7,587)	(6,875)	(7,587)
Amortization of terminated hedge included in AOCL	229	229	229	229
Change in fair value of derivatives, net of reclassifications			(123)	48
Ending Balance	$ (6,417)	$ (6,646)	$ (7,204)	$ (7,310)	$ (6,417)	$ (7,204)