Summary of Significant Accounting Policies Diluted 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, Diluted, by Common Class [Line Items]
Net income	$ 17,932	$ 17,549	$ 14,137	$ 67,223
Net income available to common stockholders	17,582	17,856	13,853	73,277
Variable Interest 3.875% Senior Convertible Debentures [Member]
Earnings Per Share, Diluted, by Common Class [Line Items]
Expense attributable to 3.875% Variable Interest Senior Convertible Debentures	0	680	0	4,608
Variable Interest 6.75% Senior Convertible Debentures [Member]
Earnings Per Share, Diluted, by Common Class [Line Items]
Expense attributable to 6.75% Variable Interest Senior Convertible Note	0	0	0	2,994
Earnings Per Share, Diluted [Member]
Earnings Per Share, Diluted, by Common Class [Line Items]
Income attributable to participating securities	$ (350)	$ (373)	$ (284)	$ (1,548)