Summarized Information on Location and Gross Amount of Derivatives on Consolidated Balance Sheets (Detail) - Not Designated as Hedging Instrument - Other Assets Liabilities - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Notional Amount		$ 230,418	$ 274,710
Fair Value		(17,931)	(10,275)
Interest Rate Swap
Derivatives, Fair Value [Line Items]
Notional Amount		200,000	200,000
Fair Value		(17,931)	(10,275)
Futures Contracts on Bonds
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	30,418	16,894
Fair Value	[1]	0	0
Futures Contracts on Equities
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	0	57,816
Fair Value	[1]	$ 0	$ 0

[1] Futures are settled daily such that their fair value is not reflected in the consolidated statements of financial position