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Fair Value Measurements - Fair Value of Certain Investments Based on NAV (Parenthetical) (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Unfunded Commitments $ 8,650 $ 10,300
Private Equity Funds [Member] | Consolidated But Not Owned [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Unfunded Commitments $ 15,517 $ 5,902