Notional and Fair Value of Non-Qualifying Hedging Instruments Hedging Instruments and Cash Flows Hedges (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative, notional value	$ 2,826,601	$ 1,813,767
Derivative, fair value	26,802	20,328
Non-qualifying hedging
Derivative [Line Items]
Derivative, notional value	2,801,604	1,763,884
Derivative, fair value	29,259	24,136
Non-qualifying hedging | Equity Futures
Derivative [Line Items]
Derivative, notional value	195,981	5,462
Derivative, fair value	(202)
Non-qualifying hedging | Bond Futures
Derivative [Line Items]
Derivative, notional value	41,008	60,697
Derivative, fair value	(59)
Non-qualifying hedging | Interest Rate Swap
Derivative [Line Items]
Derivative, notional value	221,000	221,000
Derivative, fair value	(11,248)	(2,599)
Non-qualifying hedging | Variance swaps
Derivative [Line Items]
Derivative, notional value	260	398
Derivative, fair value	(794)	(1,847)
Non-qualifying hedging | Total Return Swap
Derivative [Line Items]
Derivative, notional value	256,231	248,326
Derivative, fair value	(1,849)	(4,791)
Non-qualifying hedging | Options
Derivative [Line Items]
Derivative, notional value	1,912,124	1,043,001
Derivative, fair value	38,510	26,023
Non-qualifying hedging | Credit Default Swaps
Derivative [Line Items]
Derivative, notional value	175,000	185,000
Derivative, fair value	4,901	7,350
Designated as Hedging Instrument | Cash Flow Hedging
Derivative [Line Items]
Derivative, notional value	24,997	49,883
Derivative, fair value	(2,457)	(3,808)
Designated as Hedging Instrument | Interest Rate Swap | Cash Flow Hedging
Derivative [Line Items]
Derivative, notional value	24,997	49,883
Derivative, fair value	$ (2,457)	$ (3,808)