Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Numerator
Net income	$ 19,452	$ 16,473	$ 47,059	$ 50,024
Percentage allocated to shareholders (in hundredths)	99.60%	99.20%	99.50%	99.00%
Numerator for basic and diluted earnings per share	$ 19,374	$ 16,341	$ 46,824	$ 49,524
Denominator
Denominator for basic earnings per share - weighted-average shares outstanding (in shares)	59,211	58,598	59,049	58,441
Dilutive effect of stock options and restricted stock units (in shares)	294	389	307	544
Denominator for diluted earnings per share (in shares)	59,505	58,987	59,356	58,985
Per share income
Basic (in dollars per share)	$ 0.33	$ 0.28	$ 0.79	$ 0.85
Diluted income per share	$ 0.33	$ 0.28	$ 0.79	$ 0.84
Basic weighted-average shares outstanding (in shares)	59,211	58,598	59,049	58,441
Basic weighted-average shares outstanding and unvested restricted stock units expected to vest (in shares)	59,423	59,062	59,346	59,047
Percentage allocated to shareholders (in hundredths)	99.60%	99.20%	99.50%	99.00%