Summary of Restricted Stock Grants Since Inception (Detail) - Restricted Stock - shares	12 Months Ended			124 Months Ended	160 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted Stock Awards	156,531	174,828	132,302	1,116,348	1,580,009
Key Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted Stock Awards	121,346	138,507	95,694	711,068	1,066,615
Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted Stock Awards	35,185	36,321	36,608	405,280	513,394