CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Net income	$ 38,236,000	$ 32,151,000	$ 25,659,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization of property and equipment	24,413,000	14,815,000	13,916,000
Amortization of intangible assets	8,207,000	1,461,000	1,469,000
Accretion of closure and post-closure obligations	2,656,000	1,241,000	1,367,000
Unrealized foreign currency loss	2,427,000	2,789,000	(1,400,000)
Deferred income taxes	2,035,000	(2,637,000)	(711,000)
Share-based compensation expense	1,250,000	865,000	846,000
Unrecognized tax benefits	(480,000)	13,000	13,000
Net loss on sale of property and equipment	421,000	170,000	13,000
Amortization of debt discount	74,000
Changes in assets and liabilities (net of effect of business acquisition):
Receivables	(4,400,000)	(10,408,000)	(1,850,000)
Income taxes receivable	(1,798,000)		187,000
Other assets	116,000	(403,000)	(677,000)
Accounts payable and accrued liabilities	(2,878,000)	1,673,000	(2,172,000)
Deferred revenue	1,890,000	5,197,000	(50,000)
Accrued salaries and benefits	771,000	(424,000)	1,929,000
Income taxes payable	(389,000)	4,091,000	(1,083,000)
Closure and post-closure obligations	(1,182,000)	(955,000)	(2,282,000)
Net cash provided by operating activities	71,369,000	49,639,000	35,174,000
Cash flows from investing activities:
Business acquisition (net of cash acquired)	(460,874,000)		(10,743,000)
Purchases of property and equipment	(28,434,000)	(21,373,000)	(15,766,000)
Purchases of restricted cash and investments	(1,060,000)	(5,249,000)
Proceeds from sale of restricted cash and investments	1,023,000	5,263,000	5,000
Proceeds from sale of short term investment	654,000
Proceeds from sale of property and equipment	201,000	168,000	198,000
Net cash used in investing activities	(488,490,000)	(21,191,000)	(26,306,000)
Cash flows from financing activities:
Proceeds from issuance of long-term debt	413,962,000	9,500,000	26,000,000
Payments on long-term debt	(19,384,000)	(54,500,000)	(21,500,000)
Dividends paid	(15,532,000)	(9,978,000)	(16,432,000)
Deferred financing costs paid	(14,001,000)	(235,000)
Proceeds from exercise of stock options	1,542,000	2,461,000	1,035,000
Proceeds from public offering (net of issuance costs of $5,229)		96,431,000
Other	206,000	(1,000)	(303,000)
Net cash provided by (used in) financing activities	366,793,000	43,678,000	(11,200,000)
Effect of foreign exchange rate changes on cash	(641,000)	(306,000)	163,000
Increase (decrease) in cash and cash equivalents	(50,969,000)	71,820,000	(2,169,000)
Cash and cash equivalents at beginning of period	73,940,000	2,120,000	4,289,000
Cash and cash equivalents at end of period	$ 22,971,000	$ 73,940,000	$ 2,120,000