Quarterly Results of Operations (Tables)

12 Months Ended

Dec. 31, 2011

Quarterly Financial Information Disclosure [Abstract]

Schedule of Quarterly Financial Information [Table Text Block]

	QUARTER ENDED
	DEC. 31,	SEPT. 30,	JUNE 30,	MAR. 31,	DEC. 31,	SEPT. 30,	JUNE 30,	MAR. 31,
	2011	2011	2011	2011	2010	2010	2010	2010
Operating revenue	$169,801	$166,247	$165,027	$166,519	$164,530	$166,316	$161,789	$164,366
Income from continuing operations	34,316	35,857	38,934	30,764	32,062	35,900	33,576	28,464
Discontinued operations	6,929	5,824	53,907	4,179	4,471	5,106	6,438	7,358
Net income	41,245	41,681	92,841	34,943	36,533	41,006	40,014	35,822
Income per common share - basic (1)	0.30	0.31	0.74	0.25	0.27	0.31	0.30	0.26
Income per common share - diluted (1)	0.30	0.31	0.74	0.25	0.26	0.30	0.29	0.26

(1) The sum of quarterly financial data may vary from the annual data due to rounding.