Consolidated Statements of Common Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Capital in excess of Par	Retained earnings	Total
Balance at Dec. 31, 2012	$ 42,484	$ 628,199	$ 47,115	$ 717,798
Increase (Decrease) in Stockholders' Equity
Net income			63,445	63,445
Stock/stock units issued through:
Stock purchase and reinvestment plans	560	11,326		11,886
Dividends declared			(43,006)	(43,006)
Balance at Dec. 31, 2013	43,044	639,525	67,554	750,123
Increase (Decrease) in Stockholders' Equity
Net income			67,103	67,103
Stock/stock units issued through:
Stock purchase and reinvestment plans	435	10,018		10,453
Dividends declared			(44,381)	(44,381)
Balance at Dec. 31, 2014	43,479	649,543	90,276	783,298
Increase (Decrease) in Stockholders' Equity
Net income			56,597	56,597
Stock/stock units issued through:
Stock purchase and reinvestment plans	342	7,923		8,265
Dividends declared			(45,430)	(45,430)
Balance at Dec. 31, 2015	$ 43,821	$ 657,466	$ 101,443	$ 802,730