UNAUDITED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (LOSS) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
UNAUDITED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (LOSS) [Abstract]
Operating revenues	$ 81,416	$ 77,454	$ 161,868	$ 153,463
Operating expenses:
Cost of goods and services, exclusive of depreciation and amortization shown separately below	32,403	30,528	64,639	61,229
Selling, general and administrative, exclusive of depreciation and amortization shown separately below	16,625	16,355	33,773	32,484
Depreciation and amortization	16,595	16,071	31,983	30,042
Total operating expenses	65,623	62,954	130,395	123,755
Operating income	15,793	14,500	31,473	29,708
Other income (expense):
Interest expense	(2,065)	(2,068)	(4,112)	(4,220)
Gain on investments, net	114	30	96	178
Non-operating income, net	459	458	1,086	979
Income before taxes	14,301	12,920	28,543	26,645
Income tax expense	5,686	5,078	11,312	10,452
Net income	8,615	7,842	17,231	16,193
Other comprehensive income (loss):
Unrealized gain (loss) on interest rate hedge, net of tax	(913)	2,775	(1,260)	3,307
Comprehensive Income	$ 7,702	$ 10,617	$ 15,971	$ 19,500
Earnings per share:
Basic (in dollars per share)	$ 0.36	$ 0.33	$ 0.72	$ 0.67
Diluted (in dollars per share)	$ 0.35	$ 0.33	$ 0.71	$ 0.67
Weighted average shares outstanding, basic (in shares)	24,102	23,996	24,080	23,985
Weighted average shares outstanding, diluted (in shares)	24,320	24,078	24,271	24,055