Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Numerator
Net income	$ 15,552		$ 19,452		$ 51,432		$ 47,059
Percentage allocated to shareholders (in hundredths)	99.90%	[1]	99.60%	[1]	99.80%	[1]	99.50%	[1]
Numerator for basic and diluted earnings per share	$ 15,536		$ 19,374		$ 51,329		$ 46,824
Denominator
Denominator for basic earnings per share - weighted-average shares outstanding (in shares)	60,328		59,211		60,056		59,049
Dilutive effect of stock options and restricted stock units (in shares)	202		294		281		307
Denominator for diluted earnings per share (in shares)	60,530		59,505		60,337		59,356
Per share income
Basic (in dollars per share)	$ 0.26		$ 0.33		$ 0.85		$ 0.79
Diluted income per share: (in dollars per share)	$ 0.26		$ 0.33		$ 0.85		$ 0.79
Basic weighted-average shares outstanding (in shares)	60,328		59,211		60,056		59,049
Basic weighted-average shares outstanding and unvested restricted stock units expected to vest (in shares)	60,371		59,423		60,171		59,346
Percentage allocated to shareholders (in hundredths)	99.90%	[1]	99.60%	[1]	99.80%	[1]	99.50%	[1]

[1] Basic weighted-average shares outstanding 60,328 59,211 60,056 59,049 Basic weighted-average shares outstanding and unvested restricted share units expected to vest 60,371 59,423 60,171 59,346 Percentage allocated to shareholders 99.9% 99.6% 99.8% 99.5%