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, 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 ($0.640, $0.630 and $0.615 per share for the years ended December 31, 2013, 2012 and 2011 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 ($0.640, $0.630 and $0.615 per share for the years ended December 31, 2013, 2012 and 2011 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
Increase (Decrease) in Stockholders' Equity
Net income	47,254			47,254
Issuance of common stock	107,409	58	107,351
Issuance of common stock (in shares)		5,833
Dividends paid on common stock ($0.640, $0.630 and $0.615 per share for the years ended December 31, 2013, 2012 and 2011 respectively)	(29,619)			(29,619)
Balance at Dec. 31, 2013	$ 598,756	$ 477	$ 328,364	$ 269,915
Balance (in shares) at Dec. 31, 2013		47,741