Company Quarterly Financial Information - Schedule of Company Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 97,377	$ 97,767	$ 95,431	$ 93,431	$ 92,813	$ 96,461	$ 95,026	$ 93,512	$ 384,006	$ 377,812	$ 362,737
Net income	20,967	20,190	19,651	19,985	19,504	156,435	47,991	14,253	80,793	238,183	110,920
Noncontrolling interests	(44)	(43)	(41)	(42)	(45)	(22,729)	(272)	(17)
Dividends to preferred shareholders	(922)	(922)	(922)	(922)	(922)	(922)	(922)	(922)	(3,688)	(3,688)	(3,688)
Net income available to common shareholders	$ 20,001	$ 19,225	$ 18,688	$ 19,021	$ 18,537	$ 132,784	$ 46,797	$ 13,314	$ 76,935	$ 211,432	$ 106,846
Earnings per common share:
Net income available to common shareholders – basic	$ 0.37	$ 0.35	$ 0.34	$ 0.35	$ 0.34	$ 2.44	$ 0.86	$ 0.25	$ 1.41	$ 3.89	$ 1.96
Net income available to common shareholders – diluted	$ 0.37	$ 0.35	$ 0.34	$ 0.35	$ 0.34	$ 2.44	$ 0.86	$ 0.24	$ 1.41	$ 3.88	$ 1.96