Summary of Significant Accounting Policies Basic Earnings Per Share (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Per Share, Basic, by Common Class [Line Items]
Net income	$ 17,932	$ 17,549	$ 14,137	$ 67,223
Net income available to common stockholders	17,582	17,190	13,853	65,833
Earnings Per Share, Basic [Member]
Earnings Per Share, Basic, by Common Class [Line Items]
Income attributable to participating securities	$ (350)	$ (359)	$ (284)	$ (1,390)