Dividends Declared or Paid (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended												12 Months Ended
	Dec. 07, 2016	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	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, 2016	Dec. 31, 2015	Dec. 31, 2014
Dividends [Line Items]
Total dividend amount														$ 27,374	$ 26,275	$ 28,993
Series D cumulative redeemable preferred shares
Dividends [Line Items]
Date declared	Nov. 02, 2016
Record date	Dec. 01, 2016
Date paid	Dec. 07, 2016
Total dividend amount	$ 2,125
Preferred Stock | 7.75% Series A Cumulative Redeemable Preferred Stock
Dividends [Line Items]
Date declared		Nov. 02, 2016	Jul. 27, 2016	May 19, 2016	Feb. 16, 2016	Nov. 04, 2015	Jul. 28, 2015	May 20, 2015	Feb. 10, 2015	Oct. 28, 2014	Jul. 29, 2014	May 13, 2014	Jan. 29, 2014
Record date		Dec. 01, 2016	Sep. 01, 2016	Jun. 01, 2016	Mar. 01, 2016	Dec. 01, 2015	Sep. 01, 2015	Jun. 01, 2015	Mar. 02, 2015	Dec. 01, 2014	Sep. 02, 2014	Jun. 02, 2014	Mar. 03, 2014
Date paid		Jan. 03, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014
Total dividend amount		$ 2,589	$ 2,589	$ 2,570	$ 2,570	$ 2,570	$ 2,570	$ 2,569	$ 2,313	$ 2,313	$ 1,971	$ 1,971	$ 1,971	10,318	10,022	8,226
Preferred Stock | 8.375% Series B Cumulative Redeemable Preferred Stock
Dividends [Line Items]
Date declared		Nov. 02, 2016	Jul. 27, 2016	May 19, 2016	Feb. 16, 2016	Nov. 04, 2015	Jul. 28, 2015	May 20, 2015	Feb. 10, 2015	Oct. 28, 2014	Jul. 29, 2014	May 13, 2014	Jan. 29, 2014
Record date		Dec. 01, 2016	Sep. 01, 2016	Jun. 01, 2016	Mar. 01, 2016	Dec. 01, 2015	Sep. 01, 2015	Jun. 01, 2015	Mar. 02, 2015	Dec. 01, 2014	Sep. 02, 2014	Jun. 02, 2014	Mar. 03, 2014
Date paid		Jan. 03, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014
Total dividend amount		$ 1,225	$ 1,225	$ 1,225	$ 1,225	$ 1,225	$ 1,225	$ 1,215	$ 1,198	$ 1,198	$ 1,198	$ 1,198	$ 1,198	4,900	4,863	4,792
Preferred Stock | 8.875% Series C Cumulative Redeemable Preferred Stock
Dividends [Line Items]
Date declared		Nov. 02, 2016	Jul. 27, 2016	May 19, 2016	Feb. 16, 2016	Nov. 04, 2015	Jul. 28, 2015	May 20, 2015	Feb. 10, 2015	Oct. 28, 2014	Jul. 29, 2014	May 13, 2014	Jan. 29, 2014
Record date		Dec. 01, 2016	Sep. 01, 2016	Jun. 01, 2016	Mar. 01, 2016	Dec. 01, 2015	Sep. 01, 2015	Jun. 01, 2015	Mar. 02, 2015	Dec. 01, 2014	Sep. 02, 2014	Jun. 02, 2014	Mar. 03, 2014
Date paid		Jan. 03, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014
Total dividend amount		$ 910	$ 910	$ 910	$ 910	$ 910	$ 910	$ 910	$ 910	$ 910	$ 910	$ 910	$ 910	3,640	3,640	3,640
Preferred Stock | Series D cumulative redeemable preferred shares
Dividends [Line Items]
Date declared		Nov. 02, 2016	Jul. 27, 2016	May 19, 2016	Feb. 16, 2016	Nov. 04, 2015	Jul. 28, 2015	May 20, 2015	Feb. 10, 2015	Oct. 28, 2014	Jul. 29, 2014	May 13, 2014	Jan. 29, 2014
Record date		Dec. 01, 2016	Sep. 01, 2016	Jun. 01, 2016	Mar. 01, 2016	Dec. 01, 2015	Sep. 01, 2015	Jun. 01, 2015	Mar. 02, 2015	Dec. 01, 2014	Sep. 02, 2014	Jun. 02, 2014	Mar. 03, 2014
Date paid		Jan. 03, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014
Total dividend amount		$ 2,125	$ 2,125	$ 2,125	$ 2,125	$ 2,125	$ 1,875	$ 1,875	$ 1,875	$ 1,875	$ 1,875	$ 1,875	$ 1,255	$ 8,500	$ 7,750	$ 6,880
Common Stock
Dividends [Line Items]
Date declared		Dec. 16, 2016	Sep. 19, 2016	Jun. 15, 2016	Mar. 14, 2016	Dec. 07, 2015	Sep. 17, 2015	Jun. 22, 2015	Mar. 16, 2015	Dec. 18, 2014	Sep. 15, 2014	Jun. 12, 2014	Mar. 18, 2014
Record date		Jan. 10, 2017	Oct. 07, 2016	Jul. 08, 2016	Apr. 08, 2016	Jan. 08, 2016	Oct. 09, 2015	Jul. 10, 2015	Apr. 10, 2015	Jan. 09, 2015	Oct. 07, 2014	Jul. 11, 2014	Apr. 04, 2014
Date paid		Jan. 31, 2017	Oct. 31, 2016	Jul. 29, 2016	Apr. 29, 2016	Jan. 29, 2016	Oct. 30, 2015	Jul. 31, 2015	Apr. 30, 2015	Jan. 30, 2015	Oct. 31, 2014	Jul. 31, 2014	Apr. 30, 2014
Dividend per share		$ 0.09	$ 0.09	$ 0.09	$ 0.09	$ 0.09	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.17	$ 0.36	$ 0.63	$ 0.71
Total dividend declared		$ 8,202	$ 8,202	$ 8,202	$ 8,200	$ 8,170	$ 16,276	$ 14,775	$ 14,777	$ 14,740	$ 14,741	$ 14,738	$ 13,882	$ 32,806	$ 53,998	$ 58,101