UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME [Abstract]
Operating revenues	$ 80,452	$ 76,010
Operating expenses:
Cost of goods and services, exclusive of depreciation and amortization shown separately below	32,236	30,700
Selling, general and administrative, exclusive of depreciation and amortization shown separately below	17,149	16,129
Depreciation and amortization	15,387	13,972
Total operating expenses	64,772	60,801
Operating income	15,680	15,209
Other income (expense):
Interest expense	(2,048)	(2,152)
Gain (loss) on investments, net	(18)	148
Non-operating income, net	628	520
Income before income taxes	14,242	13,725
Income tax expense	5,626	5,374
Net income	8,616	8,351
Other comprehensive income:
Unrealized gain (loss) on interest rate hedge, net of tax	(347)	532
Comprehensive income	$ 8,269	$ 8,883
Earnings per share:
Basic and diluted earnings per share (in dollars per share)	$ 0.36	$ 0.35
Weighted average shares outstanding, basic (in shares)	24,059	23,973
Weighted average shares, diluted (in shares)	24,221	24,032