Changes in Issued and Outstanding Common Shares (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Beginning Balance	29,124,000	28,862,000	28,562,000
Net Vested Stock Grants	343,000	220,000	254,000
Employee Stock Purchase Plan	40,488	41,224	42,000
Stock Options Exercised			4,000
Ending Balance	29,507,000	29,124,000	28,862,000