Summary of Significant Accounting Policies - Summary of Newly Adopted Standards in Unaudited Consolidated Statement of Cash Flows (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities
Net cash provided by operating activities	$ 176,419	$ 145,849	[1]
Cash flows from investing activities
Purchase of short-term investments from funds held for clients	(61,268)	(137,614)	[1]
Proceeds from maturities of short-term investments from funds held for clients	54,200	95,500	[1]
Purchases of property and equipment	(71,080)	(44,264)	[1]
Net cash used in investing activities	(78,148)	(86,378)	[1]
Cash flows from financing activities
Net cash used in financing activities	(175,445)	(249,928)	[1]
Decrease in cash, cash equivalents, restricted cash and restricted cash equivalents	(77,174)	(190,457)	[1]
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	[1]
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of period	$ 909,290	908,403	[1]
As Previously Reported [Member]
Cash flows from operating activities
Net cash provided by operating activities		145,849
Cash flows from investing activities
Purchase of short-term investments from funds held for clients		(137,561)
Proceeds from maturities of short-term investments from funds held for clients		95,500
Net change in funds held for clients		229,375
Purchases of property and equipment		(44,264)
Net cash used in investing activities		143,050
Cash flows from financing activities
Net cash used in financing activities		(249,928)
Decrease in cash, cash equivalents, restricted cash and restricted cash equivalents		38,971
Cash, cash equivalents, restricted cash and restricted cash equivalents
Cash, cash equivalents, restricted cash and restricted cash equivalents, beginning of period		46,077
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of period		85,048
ASU 2016-18 [Member]
Cash flows from operating activities
Net cash provided by operating activities		145,849
Cash flows from investing activities
Purchase of short-term investments from funds held for clients		(137,614)
Proceeds from maturities of short-term investments from funds held for clients		95,500
Purchases of property and equipment		(44,264)
Net cash used in investing activities		(86,378)
Cash flows from financing activities
Net cash used in financing activities		(249,928)
Decrease in cash, cash equivalents, restricted cash and restricted cash equivalents		(190,457)
Cash, cash equivalents, restricted cash and restricted cash equivalents
Cash, cash equivalents, restricted cash and restricted cash equivalents, beginning of period		1,098,860
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of period		908,403
ASU 2016-18 [Member] | Adjustments [Member]
Cash flows from investing activities
Purchase of short-term investments from funds held for clients		(53)
Net change in funds held for clients		(229,375)
Net cash used in investing activities		(229,428)
Cash flows from financing activities
Decrease in cash, cash equivalents, restricted cash and restricted cash equivalents		(229,428)
Cash, cash equivalents, restricted cash and restricted cash equivalents
Cash, cash equivalents, restricted cash and restricted cash equivalents, beginning of period		1,052,783
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of period		$ 823,355

[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.