Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Earnings Per Share Reconciliation [Line Items]
Net income	$ 13,580	$ 14,701	$ 19,178	$ 29,214
Denominator for basic earnings per share - weighted-average number of common shares outstanding during the period	56,963,000	60,159,000	57,223,000	60,537,000
Denominator for diluted earnings per share	57,198,000	60,567,000	57,599,000	61,125,000
Basic earnings per share	$ 0.24	$ 0.24	$ 0.34	$ 0.48
Diluted earnings per share	$ 0.24	$ 0.24	$ 0.33	$ 0.48
Options to purchase of common shares	3,800,000	3,700,000	3,600,000	2,900,000
Stock Options [Member]
Earnings Per Share Reconciliation [Line Items]
Incremental common shares attributable to exercise of outstanding dilutive options	124,000	313,000	173,000	413,000
Restricted Shares, Restricted Stock Units And Phantom Stock [Member]
Earnings Per Share Reconciliation [Line Items]
Incremental common shares attributable to outstanding restricted shares, restricted stock units and phantom stock	111,000	95,000	203,000	175,000