Investment Securities (Narrative) (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) security	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) security	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Available-for-sale Securities and Held-to-maturity Securities [Abstract]
Equity securities with readily determinable fair value	$ 24,700,000		$ 24,700,000
Upward adjustments of equity securities without readily determinable fair values	25,000	$ 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	0	0	(537,000)	0
Pledged Securities, carrying value	$ 1,700,000,000	$ 1,500,000,000	$ 1,700,000,000	$ 1,500,000,000	$ 1,700,000,000
Number of securities by a single non-government sponsored issuer exceeding 10% of shareholders' equity | security	0		0