Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Numerator
Net income	$ 16,473	$ 10,629	$ 50,024	$ 26,242
Percentage allocated to shareholders (in hundredths)	99.20%	98.90%	99.00%	99.10%
Numerator for basic and diluted earnings per share	$ 16,341	$ 10,512	$ 49,524	$ 26,005
Denominator
Denominator for basic earnings per share - weighted-average shares outstanding (in shares)	58,598	57,899	58,441	57,550
Dilutive effect of stock options (in shares)	389	583	445	676
Dilutive effect of restricted stock units			99
Denominator for diluted earnings per share (in shares)	58,987	58,482	58,985	58,226
Per share income
Basic (in dollars per share)	$ 0.28	$ 0.18	$ 0.85	$ 0.45
Diluted (in dollars per share)	$ 0.28	$ 0.18	$ 0.84	$ 0.45
Basic weighted-average shares outstanding (in shares)	58,598	57,899	58,441	57,550
Basic weighted-average shares outstanding and unvested restricted share units expected to vest (in shares)	59,062	58,514	59,047	58,071
Percentage allocated to shareholders (in hundredths)	99.20%	98.90%	99.00%	99.10%