Significant Accounting Policies Earnings per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Earnings Per Share, Diluted, by Common Class, Including Two Class Method
Net income	$ 17,122	$ 15,399	$ 22,532	$ 14,879	$ 15,379	$ 18,297	$ 16,653	$ 11,887	$ 69,932	$ 62,216	$ 56,949
Weighted Average Number of Shares Outstanding, Basic									89,656	90,689	91,131
Basic EPS	$ 0.20	$ 0.17	$ 0.25	$ 0.16	$ 0.17	$ 0.20	$ 0.18	$ 0.13	$ 0.78	$ 0.69	$ 0.62
Dilutive Securities, Effect on Basic Earnings Per Share, Including Options and Restrictive Stock Units									0
Incremental Common Shares Attributable to Share-based Payment Arrangements									17
Net Income (Loss) Available to Common Stockholders, Diluted									$ 69,932
Weighted Average Number of Shares Outstanding, Diluted									89,673	90,689	91,131
Diluted EPS									$ 0.78	$ 0.69	$ 0.62