Sales Activity of Series D Preferred Shares (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Temporary Equity Disclosure [Abstract]
Aggregate purchase price	$ 100,000
Repurchase	(21,657)
Initial value of warrants and investor SARs issued to-date	(21,805)
Costs incurred	(6,834)
Total discount	(28,639)
Discount amortization to-date	27,949
Carrying amount of Series D Preferred Shares	$ 77,653	$ 81,581