Summary of Significant Accounting Policies - Summary of Newly Adopted Standards in Consolidated Statement of Cash Flows (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Cash flows from operating activities
Net cash provided by operating activities		$ 224,263	$ 184,817	[1]	$ 130,149	[1]
Cash flows from investing activities
Purchase of short-term investments from funds held for clients		(195,811)	(145,011)	[1]	(66,235)	[1]
Proceeds from maturities of short-term investments from funds held for clients		69,200	155,500	[1]	141,205	[1]
Purchases of property and equipment		(92,934)	(59,906)	[1]	(59,389)	[1]
Net cash provided by (used in) investing activities		(219,545)	(49,417)	[1]	15,581	[1]
Cash flows from financing activities
Net cash provided by (used in) financing activities		650,672	(247,796)	[1]	145,665	[1]
Increase (decrease) in cash, cash equivalents, restricted cash and restricted cash equivalents		655,390	(112,396)	[1]	291,395	[1]
Cash, cash equivalents, restricted cash and restricted cash equivalents
Cash, cash equivalents, restricted cash and restricted cash equivalents, beginning of period	[1]	986,464	1,098,860		807,465
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of period		1,641,854	986,464	[1]	1,098,860	[1]
As Previously Reported [Member]
Cash flows from operating activities
Net cash provided by operating activities			184,817		130,149
Cash flows from investing activities
Purchase of short-term investments from funds held for clients			(145,011)		(66,235)
Proceeds from maturities of short-term investments from funds held for clients			155,500		141,205
Net change in funds held for clients			112,037		(305,476)
Purchases of property and equipment			(59,906)		(59,389)
Net cash provided by (used in) investing activities			62,620		(289,895)
Cash flows from financing activities
Net cash provided by (used in) financing activities			(247,796)		145,665
Increase (decrease) in cash, cash equivalents, restricted cash and restricted cash equivalents			(359)		(14,081)
Cash, cash equivalents, restricted cash and restricted cash equivalents
Cash, cash equivalents, restricted cash and restricted cash equivalents, beginning of period		45,718	46,077		60,158
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of period			45,718		46,077
ASU 2016-18 [Member]
Cash flows from operating activities
Net cash provided by operating activities			184,817		130,149
Cash flows from investing activities
Purchase of short-term investments from funds held for clients			(145,011)		(66,235)
Proceeds from maturities of short-term investments from funds held for clients			155,500		141,205
Purchases of property and equipment			(59,906)		(59,389)
Net cash provided by (used in) investing activities			(49,417)		15,581
Cash flows from financing activities
Net cash provided by (used in) financing activities			(247,796)		145,665
Increase (decrease) in cash, cash equivalents, restricted cash and restricted cash equivalents			(112,396)		291,395
Cash, cash equivalents, restricted cash and restricted cash equivalents
Cash, cash equivalents, restricted cash and restricted cash equivalents, beginning of period		986,464	1,098,860		807,465
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of period			986,464		1,098,860
ASU 2016-18 [Member] | Adjustments [Member]
Cash flows from investing activities
Net change in funds held for clients			(112,037)		305,476
Net cash provided by (used in) investing activities			(112,037)		305,476
Cash flows from financing activities
Increase (decrease) in cash, cash equivalents, restricted cash and restricted cash equivalents			(112,037)		305,476
Cash, cash equivalents, restricted cash and restricted cash equivalents
Cash, cash equivalents, restricted cash and restricted cash equivalents, beginning of period		$ 940,746	1,052,783		747,307
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of period			$ 940,746		$ 1,052,783

[1] Amounts have been adjusted to reflect the adoption of Accounting Standards Update No. 2016-18, “Statement of Cash Flows (Topic 230): Restricted Cash (a consensus of the FASB Emerging Issues Task Force).” See Note 2 for a summary of reclassifications and adjustments.