Supplemental Disclosures of Cash Flow Information and Non-Cash Financing and Investing Activities - Significant Non-Cash Investing and Financing Activities (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Document Period End Date	Jun. 30, 2018
Cash and cash equivalents	$ 477,928,000	$ 600,296,000	$ 573,383,000	$ 515,646,000
Restricted Cash and Cash Equivalents	309,231,000	276,634,000	250,705,000	265,637,000
Cash paid during the period for:
Interest	19,112,000	18,556,000
Income taxes	65,521,000	77,343,000
Significant non-cash investing and financing activities
Other payable issued for purchased customer accounts	2,386,000	11,069,000
Estimated acquisition earn-out payables and related charges	22,071,000	282,000
Cash and Cash Equivalents inclusive of Restricted Cash [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 787,159,000	$ 876,930,000	$ 824,088,000	$ 781,283,000