Consolidated Statements of Common Stockholders' Equity (USD $) In Thousands, unless otherwise specified	Total	Common Stock	Additional Paid-in Capital	Retained Earnings
Balance at Dec. 31, 2008	$ 402,949	$ 414	$ 213,715	$ 188,820
Balance (in shares) at Dec. 31, 2008		41,447
Increase (Decrease) in Stockholders' Equity
Net income	40,554			40,554
Issuance of common stock	1,607	2	1,605
Issuance of common stock (in shares)		84
Dividends paid on common stock	(24,476)			(24,476)
Balance at Dec. 31, 2009	420,634	416	215,320	204,898
Balance (in shares) at Dec. 31, 2009		41,531
Increase (Decrease) in Stockholders' Equity
Net income	37,656			37,656
Issuance of common stock	1,989		1,989
Issuance of common stock (in shares)		136
Dividends paid on common stock	(24,753)			(24,753)
Balance at Dec. 31, 2010	435,526	416	217,309	217,801
Balance (in shares) at Dec. 31, 2010		41,667
Increase (Decrease) in Stockholders' Equity
Net income	37,712			37,712
Issuance of common stock	2,265	2	2,263
Issuance of common stock (in shares)		150
Dividends paid on common stock	(25,674)			(25,674)
Balance at Dec. 31, 2011	$ 449,829	$ 418	$ 219,572	$ 229,839
Balance (in shares) at Dec. 31, 2011		41,817