Summarized Information of Location and Gross Amount of Derivatives' Fair Value in Consolidated Balance Sheets (Detail) - Not Designated as Hedging Instrument - Other Assets Liabilities - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Notional Amount		$ 232,114	$ 200,000
Fair Value		(12,422)	(4,062)
Interest Rate Swap
Derivatives, Fair Value [Line Items]
Notional Amount		200,000	200,000
Fair Value		(12,422)	(4,062)
Futures Contracts on Bonds
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	20,726	0
Fair Value	[1]	0	0
Futures Contracts on Equities
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	11,388	0
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