Securities - Net Impairment Losses Recognized In Earnings (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Changes in Other-Than-Temporary Impairment Losses	$ (200,000)	$ (3,700,000)	$ 0
Noncredit-related Losses Recorded in / (Reclassified from) OCI	0	0	0
Net Impairment Losses Recognized in Earnings	$ (200,000)	$ (3,700,000)	$ 0