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
Earnings Per Share
Net income attributable to common shareholders	$ 50,437	$ 40,041	$ 98,904	$ 73,275
Average basic shares outstanding	58,173,000	58,909,000	58,259,000	58,777,000
Effect of dilutive stock options	1,175,000	985,000	1,198,000	939,000
Average diluted shares outstanding	59,348,000	59,894,000	59,457,000	59,716,000
Earnings per basic share	$ 0.87	$ 0.68	$ 1.70	$ 1.25
Earnings per diluted share	$ 0.85	$ 0.67	$ 1.66	$ 1.23
Average options excluded from computation of diluted earnings per share	1,000,000	2,400,000	700,000	2,900,000