Stock-Based Compensation - Summary of Nonvested Restricted Stock Awards and PSU Activity (Detail)	3 Months Ended
	Mar. 31, 2021 $ / shares shares
Time-Based Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Unvested shares of restricted stock and PSUs outstanding at beginning of period | shares	559,800
Restricted Stock Awards and PSUs, Granted | shares	102,500
Restricted Stock Awards and PSUs, Vested | shares	(2,900)
Restricted Stock Awards and PSUs, Forfeited | shares	(6,400)
Unvested shares of restricted stock and PSUs outstanding at end of period | shares	653,000.0
Unvested shares of restricted stock and PSUs outstanding, Weighted Average Grant Date Fair Value, at beginning of period | $ / shares	$ 156.48
Granted, Weighted Average Grant Date Fair Value | $ / shares	433.44
Vested, Weighted Average Grant Date Fair Value | $ / shares	142.07
Forfeited, Weighted Average Grant Date Fair Value | $ / shares	258.90
Unvested shares of restricted stock and PSUs outstanding, Weighted Average Grant Date Fair Value, at end of period | $ / shares	$ 199.03
Market-Based Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Unvested shares of restricted stock and PSUs outstanding at beginning of period | shares	1,748,500
Restricted Stock Awards and PSUs, Granted | shares	42,900
Restricted Stock Awards and PSUs, Forfeited | shares	(700)
Unvested shares of restricted stock and PSUs outstanding at end of period | shares	1,790,700
Unvested shares of restricted stock and PSUs outstanding, Weighted Average Grant Date Fair Value, at beginning of period | $ / shares	$ 115.91
Granted, Weighted Average Grant Date Fair Value | $ / shares	331.35
Forfeited, Weighted Average Grant Date Fair Value | $ / shares	331.35
Unvested shares of restricted stock and PSUs outstanding, Weighted Average Grant Date Fair Value, at end of period | $ / shares	$ 120.98
Performance-Based Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Restricted Stock Awards and PSUs, Granted | shares	52,500
Unvested shares of restricted stock and PSUs outstanding at end of period | shares	52,500	[1]
Granted, Weighted Average Grant Date Fair Value | $ / shares	$ 579.30
Unvested shares of restricted stock and PSUs outstanding, Weighted Average Grant Date Fair Value, at end of period | $ / shares	$ 579.30	[1]

[1] A maximum of 131,176 units could be awarded based upon Paycom’s Relative TSR over the applicable performance periods.