Consolidated Statements of Common Stockholders' Equity (USD $) In Thousands, unless otherwise specified	Total	Common Stock	Capital in excess of Par	Retained earnings	Accumulated comprehensive income/(loss)
Balance at Dec. 31, 2009	$ 600,150	$ 38,112	$ 551,631	$ 10,068	$ 339
Increase (Decrease) in Stockholders' Equity
Net income	47,396			47,396
Stock/stock units issued through:
Public offering	51,196	2,871	48,325
Stock purchase and reinvestment plans	11,217	594	10,623
Dividends declared	(51,996)			(51,996)
Reclassification adjustment for losses included in net income	5,814				5,814
Change in fair value of open derivative contracts for period	(6,362)				(6,362)
Income taxes	209				209
Balance at Dec. 31, 2010	657,624	41,577	610,579	5,468
Increase (Decrease) in Stockholders' Equity
Net income	54,971			54,971
Stock/stock units issued through:
Stock purchase and reinvestment plans	8,126	401	7,725
Dividends declared	(26,732)			(26,732)
Balance at Dec. 31, 2011	693,989	41,978	618,304	33,707
Increase (Decrease) in Stockholders' Equity
Net income	55,681			55,681
Stock/stock units issued through:
Stock purchase and reinvestment plans	10,401	506	9,895
Dividends declared	(42,273)			(42,273)
Balance at Dec. 31, 2012	$ 717,798	$ 42,484	$ 628,199	$ 47,115