Consolidated Statements of Common Stockholders' Equity (USD $) Share data in Thousands, unless otherwise specified	Common Stock	Additional Paid-in Capital	Retained Earnings	Total
Balance at the beginning of the period at Dec. 31, 2011	$ 418,000	$ 219,572,000	$ 229,839,000	$ 449,829,000
Balance (in shares) at Dec. 31, 2011	41,817
Increase (Decrease) in Stockholders' Equity
Net income			48,828,000	48,828,000
Issuance of common stock	1,000	1,441,000		1,442,000
Issuance of common stock (in shares)	91
Dividends paid on common stock ($0.650, $0.640 and $0.630 per share for the years ended December 31, 2014, 2013 and 2012 respectively)			(26,387,000)	(26,387,000)
Balance at the end of the period at Dec. 31, 2012	419,000	221,013,000	252,280,000	473,712,000
Balance (in shares) at Dec. 31, 2012	41,908
Increase (Decrease) in Stockholders' Equity
Net income			47,254,000	47,254,000
Issuance of common stock	58,000	107,351,000		107,409,000
Issuance of common stock (in shares)	5,833
Dividends paid on common stock ($0.650, $0.640 and $0.630 per share for the years ended December 31, 2014, 2013 and 2012 respectively)			(29,619,000)	(29,619,000)
Balance at the end of the period at Dec. 31, 2013	477,000	328,364,000	269,915,000	598,756,000
Balance (in shares) at Dec. 31, 2013	47,741
Increase (Decrease) in Stockholders' Equity
Net income			56,738,000	56,738,000
Issuance of common stock	1,000	2,194,000		2,195,000
Issuance of common stock (in shares)	65
Dividends paid on common stock ($0.650, $0.640 and $0.630 per share for the years ended December 31, 2014, 2013 and 2012 respectively)			(31,063,000)	(31,063,000)
Balance at the end of the period at Dec. 31, 2014	$ 478,000	$ 330,558,000	$ 295,590,000	$ 626,626,000
Balance (in shares) at Dec. 31, 2014	47,806