CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
OPERATING ACTIVITIES:
Net income	$ 48,705	$ 33,987
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	8,107	5,157
Stock-based compensation	27,958	12,406
Bad debt expense	1,727	3,943
Other	2,396	(1,312)
Changes in operating assets and liabilities:
Accounts receivable	(44,736)	(95,215)
Prepaid expenses and other assets	(10,597)	(7,296)
Accounts payable	(2,804)	51,855
Accrued expenses and other liabilities	7,789	3,506
Net cash provided by operating activities	38,545	7,031
INVESTING ACTIVITIES:
Purchases of property and equipment	(10,383)	(8,357)
Capitalized software development costs	(4,340)	(2,479)
Net cash used in investing activities	(14,723)	(10,836)
FINANCING ACTIVITIES:
Repayment on line of credit	(27,000)
Payment of debt financing costs		(153)
Payment of financing obligations		(677)
Proceeds from exercise of stock options	7,165	1,768
Proceeds from employee stock purchase plan	7,014	2,294
Taxes paid related to net settlement of restricted stock awards	(602)	(29)
Net cash provided by (used in) financing activities	(13,423)	3,203
Increase (decrease) in cash and cash equivalents	10,399	(602)
Cash and cash equivalents—Beginning of period	155,950	133,400
Cash and cash equivalents—End of period	166,349	132,798
SUPPLEMENTAL CASH FLOW INFORMATION:
Capitalized assets financed by accounts payable	6,624	945
Tenant improvements paid by lessor	1,017	640
Asset retirement obligation	386
Debt financing costs included in debt, net		1,153
Stock-based compensation included in capitalized software development costs	$ 1,192	$ 453