Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Numerator
Net income	$ 21,217	$ 6,345	$ 33,552	$ 15,613
Percentage allocated to shareholders (in hundredths)	98.90%	99.20%	98.90%	99.20%
Numerator for basic and diluted earnings per share	$ 20,984	$ 6,294	$ 33,183	$ 15,488
Denominator
Denominator for basic earnings per share - weighted-average shares outstanding (in shares)	58,422,000	57,682,000	58,361,000	57,373,000
Dilutive effect of stock options (in shares)	454,000	667,000	472,000	729,000
Denominator for diluted earnings per share (in shares)	58,876,000	58,349,000	58,833,000	58,102,000
Per share income
Basic (in dollars per share)	$ 0.36	$ 0.11	$ 0.57	$ 0.27
Diluted (in dollars per share)	$ 0.36	$ 0.11	$ 0.56	$ 0.27
Basic weighted-average shares outstanding (in shares)	58,422,000	57,682,000	58,361,000	57,373,000
Basic weighted-average shares outstanding and unvested restricted share units expected to vest (in shares)	59,096,000	58,155,000	59,039,000	57,847,000
Percentage allocated to shareholders (in hundredths)	98.90%	99.20%	98.90%	99.20%
Stock options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)		100,000		100,000