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CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Income Statement [Abstract] |  |  |
Revenues (gross billings of $20.174 billion, $17.933 billion, and $15.806 billion, less worksite employee payroll cost of $16.874 billion, $14.992 billion, and $13.202 billion, respectively) | $ 3,300,223 | $ 2,941,347 | $ 2,603,614
Direct costs: |  |  |
Payroll taxes, benefits and workers’ compensation costs | 2,727,492 | 2,449,737 | 2,165,747
Gross profit | 572,731 | 491,610 | 437,867
Operating expenses: |  |  |
Salaries, wages and payroll taxes | 259,531 | 229,589 | 211,060
Stock-based compensation | 24,345 | 16,643 | 13,345
Commissions | 22,773 | 19,288 | 18,479
Advertising | 16,686 | 16,447 | 15,980
General and administrative expenses | 101,273 | 86,693 | 84,259
Impairment charges and other | 0 | 0 | 10,480
Depreciation and amortization | 18,182 | 16,644 | 18,565
Total operating expenses | 442,790 | 385,304 | 372,168
Operating income | 129,941 | 106,306 | 65,699
Other income (expense): |  |  |
Interest Income | 3,413 | 1,267 | 379
Interest Expense | (3,213) | (2,396) | (459)
Income before income tax expense | 130,141 | 105,177 | 65,619
Income tax expense | 45,739 | 39,186 | 26,229
Net income | 84,402 | 65,991 | 39,390
Less distributed and undistributed earnings allocated to participating securities | 1,517 | 1,496 | 981
Net income allocated to common shares | $ 82,885 | $ 64,495 | $ 38,409
Basic net income per share of common stock (in dollars per share) | $ 2.02 | $ 1.55 | $ 0.79
Diluted net income per share of common stock (in dollars per share) | $ 2.01 | $ 1.54 | $ 0.79

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