Hierarchy of Assets and Liabilities Measured at Fair Value on Recurring Basis (Parenthetical) (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments measured at NAV	$ 5,532,002	$ 5,795,209
Unfunded Commitments	0	0
Separate Accounts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments measured at NAV	5,531,025	5,793,834
Unfunded Commitments	$ 0	0
Redemption Frequency	None
Separate Accounts | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Redemption Notice Period	0 days
Separate Accounts | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Redemption Notice Period	0 days
Limited Partnership | Private Equity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments measured at NAV	$ 977	1,375
Unfunded Commitments	$ 0	$ 0
Redemption Frequency	None