Supplemental Disclosures of Cash Flow Information and Non-Cash Financing and Investing Activities - Significant Non-Cash Investing and Financing Activities (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Document Period End Date	Sep. 30, 2018
Cash and cash equivalents	$ 422,971,000	$ 546,520,000	$ 573,383,000	$ 515,646,000
Restricted Cash and Cash Equivalents	301,716,000	276,687,000	250,705,000	265,637,000
Cash paid during the period for:
Interest	32,896,000	32,504,000
Income taxes	21,380,000	110,853,000
Significant non-cash investing and financing activities
Other payable issued for purchased customer accounts	3,339,000	11,395,000
Estimated acquisition earn-out payables and related charges	51,717,000	1,332,000
Cash and Cash Equivalents inclusive of Restricted Cash [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 724,687,000	$ 823,207,000	$ 824,088,000	$ 781,283,000