Recent Accounting Developments - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Reclassification from accumulated other comprehensive income to retained earnings	$ 17,000
Gain (loss) on bank investment securities	(9,431)
ASU 2016-01 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Gain (loss) on bank investment securities	$ 9,000
ASU 2016-02 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Minimum lease payments under non cancelable operating lease		$ 429,000
Lessee, operating lease term	12 months