Net Income Per Common Share (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Net Income Per Common Share [Abstract]
Average common shares outstanding during the period for basic computation	139,108,000	138,114,000	138,935,000	137,971,000
Dilutive effect of employee stock-based compensation	735,000	667,000	642,000	547,000
Average common shares outstanding during the period for diluted computation	139,843,000	138,781,000	139,577,000	138,518,000
Employee stock options excluded from calculations of diluted net income per share	878,789	933,800	878,789	933,800