Debt and Short-Term Borrowings - Total Debt (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Note payable	$ 400	$ 1,000
Total debt	569,731	616,740
Note Payable due on August 31, 2019 | Notes Payable
Debt Instrument [Line Items]
Total debt	$ 0	584
Fixed interest rate	7.50%
Note Payable due on April 30, 2021
Debt Instrument [Line Items]
Note payable	$ 360	418
Term A due on April 17, 2018 | Credit Facility
Debt Instrument [Line Items]
Credit facility	$ 0	$ 26,690
Term A due on April 17, 2018 | Credit Facility | LIBOR
Debt Instrument [Line Items]
Margin interest rate	2.25%	2.25%
Term A due January 17, 2020 | Credit Facility
Debt Instrument [Line Items]
Credit facility	$ 193,808	$ 200,653
Term A due January 17, 2020 | Credit Facility | LIBOR
Debt Instrument [Line Items]
Margin interest rate	2.50%	2.50%
Term B due on April 17, 2020 | Credit Facility
Debt Instrument [Line Items]
Credit facility	$ 375,563	$ 376,395
Term B due on April 17, 2020 | Credit Facility | LIBOR
Debt Instrument [Line Items]
Margin interest rate	2.50%	2.50%
Minimum variable rate	0.75%	0.75%
Senior Secured Revolving Credit Facility | Credit Facility
Debt Instrument [Line Items]
Note payable	$ 0	$ 12,000
Senior Secured Revolving Credit Facility | Credit Facility | LIBOR
Debt Instrument [Line Items]
Margin interest rate	2.50%	2.50%