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
Numerator: [Abstract]:
Net income	$ 97,628	$ 69,007	$ 181,243	$ 133,781
Earnings allocated to participating securities	(977)	(559)	(1,716)	(1,020)
Numerator for basic earnings per share - income available to common shareholders	96,651	68,448	179,527	132,761
Effect of reallocating undistributed earnings of participating securities	3	2	5	3
Numerator for diluted earnings per share - income available to common shareholders	$ 96,654	$ 68,450	$ 179,532	$ 132,764
Denominator: [Abstract]
Weighted average shares outstanding (in shares)	68,152,242	68,451,428	68,214,648	68,419,699
Less: Participating securities included in weighted average shares outstanding (in shares)	(679,577)	(552,945)	(641,368)	(519,420)
Denominator for basic earnings per common share (in shares)	67,472,665	67,898,483	67,573,280	67,900,279
Dilutive effect of employee stock compensation plans (in shares)	272,163	271,002	274,379	272,903
Denominator for diluted earnings per common share (in shares)	67,744,828	68,169,485	67,847,659	68,173,182
Basic earnings per share (per share)	$ 1.43	$ 1.01	$ 2.66	$ 1.96
Diluted earnings per share (per share)	$ 1.43	$ 1.00	$ 2.65	$ 1.95
Excludes employee stock options with exercise prices greater than current market price.	366,407	785,686	361,558	771,343