Hierarchy of Assets and Liabilities Measured at Fair Value on Recurring Basis (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments measured at NAV	$ 5,796,266	$ 5,730,860
Unfunded Commitments	102	0
Separate Accounts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments measured at NAV	5,723,657	5,659,950
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
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments measured at NAV	$ 72,609	70,910
Unfunded Commitments	102	0
Limited Partnership | Private Equity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments measured at NAV	1,704	1,759
Unfunded Commitments	$ 0	0
Redemption Frequency	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
Limited Partnership | Hedge Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments measured at NAV	$ 70,905	69,151
Unfunded Commitments	$ 102	$ 0
Redemption Frequency	Quarterly
Limited Partnership | Hedge Funds | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Redemption Notice Period	60 days
Limited Partnership | Hedge Funds | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Redemption Notice Period	65 days