Hierarchy of Assets and Liabilities Measured at Fair Value on Recurring Basis (Parenthetical) (Detail) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments measured at NAV		$ 5,562,969	$ 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		5,561,785	5,793,834
Unfunded Commitments		$ 0	0
Redemption Frequency		Daily
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		$ 1,184	1,375
Unfunded Commitments		$ 0	$ 0
Redemption Frequency	[1]	None
Limited Partnership | Private Equity | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Redemption Notice Period		0 days
Limited Partnership | Private Equity | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Redemption Notice Period		0 days

[1] The general partner would have to consent to any sale of limited partnership interest. The general partner has no obligation to liquidate any limited partner interest prior to the final dissolution of the Partnership.