Notional and Fair Value of Non-Qualifying Hedging Instruments Hedging Instruments and Cash Flows Hedges (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative, notional value	$ 3,392,774	$ 3,999,755
Derivative, fair value	(4,488)	1,713
Non-qualifying hedging
Derivative [Line Items]
Derivative, notional value	3,342,891	3,949,871
Derivative, fair value	(1,648)	4,715
Non-qualifying hedging | Future | Short
Derivative [Line Items]
Derivative, notional value	5,423	25,157
Non-qualifying hedging | Future | Long
Derivative [Line Items]
Derivative, notional value	52,168	55,071
Non-qualifying hedging | Interest Rate Swap
Derivative [Line Items]
Derivative, notional value	251,000	251,000
Derivative, fair value	(3,429)	(2,799)
Non-qualifying hedging | Variance swaps
Derivative [Line Items]
Derivative, notional value	540	540
Derivative, fair value	(3,464)	(1,835)
Non-qualifying hedging | Total Return Swap
Derivative [Line Items]
Derivative, notional value	800,910	1,405,253
Derivative, fair value	(8,154)	(16,487)
Non-qualifying hedging | Options
Derivative [Line Items]
Derivative, notional value	2,002,850	2,002,850
Derivative, fair value	7,616	24,525
Non-qualifying hedging | Credit Default Swaps
Derivative [Line Items]
Derivative, notional value	230,000	210,000
Derivative, fair value	5,783	1,311
Designated as Hedging Instrument | Cash Flow Hedging
Derivative [Line Items]
Derivative, notional value	49,883	49,883
Derivative, fair value	(2,840)	(3,002)
Designated as Hedging Instrument | Interest Rate Swap | Cash Flow Hedging
Derivative [Line Items]
Derivative, notional value	49,883	49,883
Derivative, fair value	$ (2,840)	$ (3,002)