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Fair Value Measurements - Fair Value of Certain Investments Based on NAV (Detail) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Fair Value $ 68,247 $ 80,378
Unfunded Commitments 8,650 10,300
Hedge Funds [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Fair Value $ 332 $ 2,517
Investments Redeemable, Redemption Notice Period 60 days  
Hedge Funds [Member] | Monthly [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Percent 100.00% 5.00%
Hedge Funds [Member] | Quarterly [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Percent   92.00%
Hedge Funds [Member] | Weekly [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Percent   3.00%
Hedge Funds [Member] | Minimum [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Redemption Notice Period   30 days
Hedge Funds [Member] | Maximum [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Redemption Notice Period   60 days
Funds of Funds [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Fair Value $ 25 $ 528
Investments Redeemable, Redemption Notice Period 90 days  
Funds of Funds [Member] | Monthly [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Percent   97.00%
Funds of Funds [Member] | Quarterly [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Percent 100.00% 3.00%
Funds of Funds [Member] | Minimum [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Redemption Notice Period   30 days
Funds of Funds [Member] | Maximum [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Redemption Notice Period   90 days
Other [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Fair Value $ 572 $ 595
Other [Member] | Monthly [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Percent 94.00% 94.00%
Other [Member] | Daily [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Percent 6.00% 6.00%
Other [Member] | Minimum [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Redemption Notice Period 30 days 30 days
Other [Member] | Maximum [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Redemption Notice Period 60 days 60 days
Debt Funds [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Fair Value $ 7 $ 6
Investments Redeemable, Redemption Notice Period 30 days 30 days
Debt Funds [Member] | Daily [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Percent 100.00% 100.00%
Equity Funds [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Fair Value $ 53 $ 53
Equity Funds [Member] | Monthly [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Percent 49.00% 50.00%
Equity Funds [Member] | Quarterly [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Percent 32.00% 32.00%
Equity Funds [Member] | Daily [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Percent 19.00% 18.00%
Equity Funds [Member] | Minimum [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Redemption Notice Period 30 days 30 days
Equity Funds [Member] | Maximum [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Investments Redeemable, Redemption Notice Period 90 days 90 days
Private Equity Funds [Member] | Equity Growth [Member]    
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items]    
Fair Value $ 67,258 $ 76,679
Unfunded Commitments $ 8,650 $ 10,300
% of Fair Value Not Redeemable 100.00% 100.00%
Estimated Liquidation Period of Investments Not Redeemable, % Next 5 Years 18.00% 16.00%
Estimated Liquidation Period of Investments Not Redeemable, % 5-10 Years 39.00% 38.00%
Estimated Liquidation Period of Investments Not Redeemable, % Thereafter 43.00% 46.00%