Dividends Declared and Paid on Preferred Shares (Detail) - USD ($)		3 Months Ended	6 Months Ended
	Mar. 19, 2018	Jun. 30, 2018	Jun. 30, 2018
Dividends [Line Items]
Total dividend amount		$ 0	$ 6,397,000
Preferred Stock | 7.75% Series A Cumulative Redeemable Preferred Stock
Dividends [Line Items]
Date declared			Mar. 13, 2018
Record date			Mar. 23, 2018
Date paid			Mar. 30, 2018
Total dividend amount			$ 2,589,000
Preferred Stock | 8.375% Series B Cumulative Redeemable Preferred Stock
Dividends [Line Items]
Date declared			Mar. 13, 2018
Record date			Mar. 23, 2018
Date paid			Mar. 30, 2018
Total dividend amount			$ 1,225,000
Preferred Stock | 8.875% Series C Cumulative Redeemable Preferred Stock
Dividends [Line Items]
Date declared			Mar. 13, 2018
Record date			Mar. 23, 2018
Date paid			Mar. 30, 2018
Total dividend amount			$ 910,000
Preferred Stock | Series D Cumulative Redeemable Preferred Shares
Dividends [Line Items]
Date declared	Mar. 13, 2018		Mar. 13, 2018
Record date	Mar. 23, 2018		Mar. 23, 2018
Date paid			Mar. 30, 2018
Total dividend amount	$ 1,665,000		$ 1,665,000