Notional and Fair Value of Non-Qualifying Hedging Instruments Hedging Instruments and Cash Flows Hedges (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative, notional value	$ 1,732,958	$ 1,813,767
Derivative, fair value	33,500	20,328
Non-qualifying hedging
Derivative [Line Items]
Derivative, notional value	1,707,961	1,763,884
Derivative, fair value	35,516	24,136
Non-qualifying hedging | Future | Short
Derivative [Line Items]
Derivative, notional value	8,435	5,462
Derivative, fair value	(80)
Non-qualifying hedging | Future | Long
Derivative [Line Items]
Derivative, notional value	47,993	60,697
Derivative, fair value	216
Non-qualifying hedging | Interest Rate Swap
Derivative [Line Items]
Derivative, notional value	221,000	221,000
Derivative, fair value	(7,475)	(2,599)
Non-qualifying hedging | Variance swaps
Derivative [Line Items]
Derivative, notional value	318	398
Derivative, fair value	106	(1,847)
Non-qualifying hedging | Total Return Swap
Derivative [Line Items]
Derivative, notional value	241,325	248,326
Derivative, fair value	(3,566)	(4,791)
Non-qualifying hedging | Options
Derivative [Line Items]
Derivative, notional value	1,013,890	1,043,001
Derivative, fair value	39,790	26,023
Non-qualifying hedging | Credit Default Swaps
Derivative [Line Items]
Derivative, notional value	175,000	185,000
Derivative, fair value	6,525	7,350
Designated as Hedging Instrument | Cash Flow Hedging
Derivative [Line Items]
Derivative, notional value	24,997	49,883
Derivative, fair value	(2,016)	(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	$ (2,016)	$ (3,808)