Balance Sheets (Parenthetical) (Detail) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Condensed Balance Sheet Statements, Captions [Line Items]
Common shares, par value	$ 0.01	$ 0.01
Common shares, authorized	200,000,000	200,000,000
Common shares, issued	37,551,191	36,446,691
Common shares, outstanding	37,551,191	36,446,691
Parent Company
Condensed Balance Sheet Statements, Captions [Line Items]
Common shares, par value	$ 0.01	$ 0.01
Common shares, authorized	200,000,000	200,000,000
Common shares, issued	37,551,191	36,446,691
Common shares, outstanding	37,551,191	36,446,691