Fair Value Measurements - Summary of Changes in Fair Value of Company's Level 3 Assets and Liabilities (Parenthetical) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equities [Member]
Fair Value of Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation [Line Items]
Net unrealized gains	$ 12	$ 243	$ 14
Contingent Consideration Liability [Member]
Fair Value of Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation [Line Items]
Net unrealized losses	$ 6,582	2,658
Contingent Consideration Liability [Member] | Edgewater Acquisition [Member]
Fair Value of Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation [Line Items]
Net realized losses		$ 30,067