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, 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 ($0.630, $0.615 and $0.595 per share for the years ended December 31, 2012, 2011 and 2010 respectively)	(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 ($0.630, $0.615 and $0.595 per share for the years ended December 31, 2012, 2011 and 2010 respectively)	(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
Increase (Decrease) in Stockholders' Equity
Net income	48,828			48,828
Issuance of common stock	1,442	1	1,441
Issuance of common stock (in shares)		91
Dividends paid on common stock ($0.630, $0.615 and $0.595 per share for the years ended December 31, 2012, 2011 and 2010 respectively)	(26,387)			(26,387)
Balance at Dec. 31, 2012	$ 473,712	$ 419	$ 221,013	$ 252,280
Balance (in shares) at Dec. 31, 2012		41,908