Earnings Per Share (Details) (USD $) In Thousands, except Share data	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010
Numerator: [Abstract]:
Net income	$ 69,007		$ 63,522		$ 133,781		$ 123,655
Earnings allocated to participating securities	(559)		(434)		(1,020)		(767)
Numerator for basic earnings per share - income available to common shareholders	68,448		63,088		132,761		122,888
Effect of reallocating undistributed earnings of participating securities	2		1		3		2
Numerator for diluted earnings per share - income available to common shareholders	$ 68,450		$ 63,089		$ 132,764		$ 122,890
Denominator: [Abstract]
Weighted average shares outstanding (in shares)	68,451,428		68,069,864		68,419,699		68,018,225
Less: Participating securities included in weighted average shares outstanding (in shares)	(552,945)		(464,057)		(519,420)		(418,876)
Denominator for basic earnings per common share (in shares)	67,898,483		67,605,807		67,900,279		67,599,349
Dilutive effect of employee stock compensation plans (in shares)	271,002	[1]	274,780	[1]	272,903	[1]	236,257	[1]
Denominator for diluted earnings per common share (in shares)	68,169,485		67,880,587		68,173,182		67,835,606
Basic earnings per share (per share)	$ 1.01		$ 0.93		$ 1.96		$ 1.82
Diluted earnings per share (per share)	$ 1.00		$ 0.93		$ 1.95		$ 1.81
Excludes employee stock options with exercise prices greater than current market price.	785,686		601,361		771,343		1,018,503

[1] Excludes employee stock options with exercise prices greater than current market price.