UNAUDITED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (LOSS) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
UNAUDITED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (LOSS) [Abstract]
Operating revenues	$ 92,571	$ 84,287
Operating expenses:
Cost of goods and services, exclusive of depreciation and amortization shown separately below	31,762	30,691
Selling, general and administrative, exclusive of depreciation and amortization shown separately below	21,758	18,733
Depreciation and amortization	17,739	16,337
Total operating expenses	71,259	65,761
Operating income	21,312	18,526
Other income (expense):
Interest expense	(1,619)	(1,915)
Gain on investments, net	88	102
Non-operating income, net	468	432
Income before income taxes	20,249	17,145
Income tax expense	6,368	6,859
Net income	13,881	10,286
Other comprehensive income:
Unrealized loss on interest rate hedge, net of tax	(1,048)	(907)
Comprehensive income	$ 12,833	$ 9,379
Earnings per share:
Basic (in dollars per share)	$ 0.29	$ 0.21
Diluted (in dollars per share)	$ 0.28	$ 0.21
Weighted average shares outstanding, basic (in shares)	48,563	48,306
Weighted average shares outstanding, diluted (in shares)	49,249	48,902