UNAUDITED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
UNAUDITED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (LOSS) [Abstract]
Operating revenues	$ 85,701	$ 81,416	$ 169,989	$ 161,868
Operating expenses:
Cost of goods and services, exclusive of depreciation and amortization shown separately below	30,280	32,403	60,970	64,639
Selling, general and administrative, exclusive of depreciation and amortization shown separately below	19,008	16,625	37,742	33,773
Depreciation and amortization	17,663	16,595	34,001	31,983
Total operating expenses	66,951	65,623	132,713	130,395
Operating income	18,750	15,793	37,276	31,473
Other income (expense):
Interest expense	(1,940)	(2,065)	(3,855)	(4,112)
Gain on investments, net	98	114	200	96
Non-operating income, net	442	459	874	1,086
Income before income taxes	17,350	14,301	34,495	28,543
Income tax expense	6,876	5,686	13,735	11,312
Net income	10,474	8,615	20,760	17,231
Other comprehensive income:
Unrealized gain (loss) on interest rate hedge, net of tax	326	(913)	(581)	(1,260)
Comprehensive income	$ 10,800	$ 7,702	$ 20,179	$ 15,971
Earnings per share:
Basic (in dollars per share)	$ 0.43	$ 0.36	$ 0.86	$ 0.72
Diluted (in dollars per share)	$ 0.43	$ 0.35	$ 0.85	$ 0.71
Weighted average shares outstanding, basic (in shares)	24,190	24,102	24,172	24,080
Weighted average shares outstanding, diluted (in shares)	24,502	24,320	24,463	24,271