Sales Activity of Series D Preferred Shares (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 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	25,950
Carrying amount of Series D preferred shares	$ 75,654	$ 81,581