CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME [Abstract]
Operating revenues	$ 342,485	$ 326,946	$ 308,942
Operating expenses
Cost of goods and services, exclusive of depreciation and amortization shown separately below	121,330	129,743	125,140
Selling, general and administrative, exclusive of depreciation and amortization shown below	76,367	69,370	67,673
Depreciation and amortization	70,702	65,890	60,722
Total operating expenses	268,399	265,003	253,535
Operating income	74,086	61,943	55,407
Other income (expense)
Interest expense	(7,355)	(8,148)	(8,468)
Gain on investments, net	105	208	756
Non-operating income, net	1,754	2,031	1,769
Income before income taxes	68,590	56,034	49,464
Income tax expense	27,726	22,151	19,878
Net income	40,864	33,883	29,586
Other comprehensive income:
Unrealized gain (loss) on interest rate hedge, net of tax	(707)	(1,472)	3,457
Comprehensive income	$ 40,157	$ 32,411	$ 33,043
Earning per share:
Basic (in dollars per share)	$ 0.84	$ 0.70	$ 0.62
Diluted (in dollars per share)	$ 0.83	$ 0.70	$ 0.61
Weighted average shares outstanding, basic (in shares)	48,388	48,198	48,002
Weighted average shares outstanding, diluted (in shares)	49,024	48,720	48,230