Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Numerator [Abstract]
Net income	$ 11,820	$ 11,869
Basic earnings per share [Abstract]
Weighted average shares (in shares)	15,539,000	15,407,000
Diluted earnings per share [Abstract]
Effect of dilutive stock options (in shares)	53,000	49,000
Adjusted weighted average shares (in shares)	15,592,000	15,456,000
Earnings per share [Abstract]
Basic earnings per share (in dollars per share)	$ 0.76	$ 0.77
Diluted earnings per share (in dollars per share)	$ 0.76	$ 0.77
Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Options excluded from diluted calculations (in shares)	88,246	248,555