Series D Preferred Shares - Additional Information (Detail)			1 Months Ended		12 Months Ended
	Jun. 22, 2017 USD ($) shares	Dec. 07, 2016 USD ($) shares	Mar. 31, 2018 shares	Mar. 16, 2018 USD ($)	Dec. 31, 2017 USD ($) $ / shares	Oct. 18, 2017 USD ($)	Oct. 17, 2017 USD ($)	Oct. 11, 2017 USD ($)	Dec. 31, 2016 $ / shares	Dec. 31, 2015 shares	Sep. 30, 2015 USD ($) Covenant	Oct. 01, 2012 USD ($) $ / shares shares
Temporary Equity [Line Items]
Aggregate purchase price under Securities Purchase Agreement					$ 100,000,000
Stock Appreciation Rights | shares												6,735,667
Percentage of aggregate amount of common shares issuable upon exercise of warrants					5.00%
Subsequent Event
Temporary Equity [Line Items]
Net proceeds from portion of assets sold				$ 4,863,000
ARS VI
Temporary Equity [Line Items]
Aggregate purchase price under Securities Purchase Agreement												$ 100,000,000
ARS VI | Warrants and investor SARs
Temporary Equity [Line Items]
Class of warrant and right exercise of purchase value.								$ 20,500,000
Fair value adjustment of warrants and investor rights obligation						$ 0
Warrants and rights outstanding							$ 0
Preferred Series D
Temporary Equity [Line Items]
Cumulative redeemable preferred shares, shares issued | shares	3,133,720	3,536,000								4,000,000
Preferred shares with warrants | shares												9,931,000
Percentage of outstanding Series D preferred shares or warrants					10.00%
Preferred Shares, Dividend Rate					7.50%
Preferred Shares, Dividend Rate increase per each anniversary					0.50%
Liquidation preference per share to October 1, 2017 | $ / shares					$ 26.25
Liquidation preference per share after five years | $ / shares					$ 25.00
Net proceeds from sale,transfer, repayment or other disposition of investments					$ 38,941,000
Shares repurchase and cancel, shares | shares	402,280	464,000
Repurchase shares price	$ 10,057,000	$ 11,600,000
Cumulative redeemable preferred shares, shares outstanding | shares	3,133,720	3,536,000								4,000,000
Preferred Series D | Scenario Forecast
Temporary Equity [Line Items]
Cumulative redeemable preferred shares, shares outstanding | shares			2,939,190
Shares redeem and cancel, shares | shares			194,530
Preferred Series D | October 1, 2015
Temporary Equity [Line Items]
Preferred Shares, Dividend Rate					8.50%
Preferred Series D | Prior To October 1, 2017
Temporary Equity [Line Items]
Redemption price | $ / shares					$ 26.25
Preferred Series D | After October 1, 2017
Temporary Equity [Line Items]
Redemption price | $ / shares					25.00
Preferred Series D | October 1, 2019
Temporary Equity [Line Items]
Redemption price | $ / shares					25.00
Preferred Series D | ARS VI
Temporary Equity [Line Items]
Cumulative redeemable preferred shares, shares issued | shares												4,000,000
Cumulative Redeemable Preferred Shares, par value | $ / shares												$ 0.01
Number of covenants | Covenant											2
Consideration paid under Securities Purchase Agreement											$ 450,000
Series E Cumulative Redeemable Preferred Stock
Temporary Equity [Line Items]
Cumulative Redeemable Preferred Shares, par value | $ / shares					$ 0.01				$ 0.01