Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities:
Net income	$ 54,989	$ 40,054	$ 37,636
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization expenses related to property and equipment	33,254	27,515	24,882
Amortization of intangible assets	24,342	22,555	20,164
Amortization of deferred financing costs	564	2,258	2,083
Stock-based compensation expense	21,507	24,143	13,277
Deferred income taxes	(3,504)	(20,408)	(7,368)
(Gain) / Loss on disposal of assets		209	(948)
Change in fair value of contingent consideration	(35)	(2,865)
Release of estimated liability	(8,436)
Changes in operating assets and liabilities:
Accounts receivable	(17,312)	(6,976)	(3,554)
Prepaid expenses	(3,381)	(1,463)	(2,399)
Other current assets	596	165	2,066
Other assets	245	124	234
Income taxes receivable / (payable)	(16,925)	1,462	(7,227)
Accounts payable, accrued expenses and other liabilities	11,181	(340)	12,116
Estimated liability for appeals	(628)	32	(2,323)
Net cash provided by operating activities	96,457	86,464	88,639
Investing activities:
Acquisition of a business, net of cash acquired		(171,321)	(20,678)
Proceeds from sale of cost basis investment			2,496
Purchases of property and equipment	(11,264)	(17,318)	(13,703)
Investment in capitalized software	(19,149)	(15,725)	(7,316)
Net cash used in investing activities	(30,413)	(204,364)	(39,201)
Financing activities:
Proceeds from credit facility		42,204
Payments for deferred financing costs		(2,269)
Proceeds from exercise of stock options	38,362	2,720	2,940
Payments of tax withholdings on behalf of employees for net-share settlements	(2,818)	(3,161)	(1,475)
Payments on capital lease obligations		(143)	(44)
Purchases of treasury stock	(5,955)	(14,137)	(20,470)
Net cash provided by/(used in) financing activities	29,589	25,214	(19,049)
Net increase/(decrease) in cash and cash equivalents	95,633	(92,686)	30,389
Cash and Cash Equivalents
Cash and cash equivalents at beginning of year	83,313	175,999	145,610
Cash and cash equivalents at end of period	178,946	83,313	175,999
Supplemental disclosure of cash flow information:
Cash paid for income taxes, net of refunds	22,225	17,995	20,326
Cash paid for interest	10,326	9,944	6,196
Supplemental disclosure of non-cash activities:
Change in balance of accrued property and equipment purchases	$ 1,305	$ 51	$ 684