Investment Securities (Narrative) (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Available-for-sale Securities and Held-to-maturity Securities [Abstract]
Impairment of equity securities without readily determinable fair values	$ (330,000)	$ 0	$ (867,000)	$ 0
Equity securities with readily determinable fair value	26,600,000		26,600,000
Upward adjustments of equity securities without readily determinable fair values	0	0	156,000	0
Downward adjustments of equity securities without readily determinable fair values	0		0
Impairment of equity securities without readily determinable fair values	330,000	0	867,000	0
Pledged Securities, carrying value	$ 1,700,000,000	$ 1,600,000,000	$ 1,700,000,000	$ 1,600,000,000	$ 1,700,000,000
Number of securities by a single non-government sponsored issuer exceeding 10% of shareholders' equity | security	0		0