Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Numerator
Net income	$ 12,993		$ 12,335
Percentage allocated to shareholders (in hundredths)	99.40%	[1]	98.80%	[1]
Numerator for basic and diluted earnings per share	$ 12,915		$ 12,187
Denominator
Denominator for basic earnings per share - weighted-average shares outstanding (in shares)	58,941		58,299
Dilutive effect of stock options and restricted stock units (in shares)	334		490
Denominator for diluted earnings per share (in shares)	59,275		58,789
Per share income
Basic (in dollars per share)	$ 0.22		$ 0.21
Diluted income per share (in dollars per share)	$ 0.22		$ 0.21
Basic weighted-average shares outstanding (in shares)	58,941		58,299
Basic weighted-average shares outstanding and unvested restricted stock units expected to vest (in shares)	59,286		58,982
Percentage allocated to shareholders (in hundredths)	99.40%	[1]	98.80%	[1]

[1] Basic weighted-average shares outstanding Quarters Ended March 31, 2012 2011 58941 58299 Basic weighted-average shares outstanding and unvested restricted share units expected to vest Quarters Ended March 31, 2012 2011 59286 58982 Percentage allocated to shareholders Quarters Ended March 31, 2012 2011 99.4% 98.8%