Fair Value Measurements - Changes in Level 3 Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) - Fair Value Measurements, Recurring [Member] - Significant Unobservable Inputs (Level 3) [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Privately Issued Mortgage-Backed Securities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 22	$ 28
Total gains (losses) realized/unrealized:
Settlements	(6)	(1)
Ending Balance	16	27
Other Assets and Other Liabilities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	7,712	8,303
Total gains (losses) realized/unrealized:
Included in earnings	16,546	8,130
Transfers out of Level 3	(16,825)	(7,673)
Ending Balance	7,433	8,760
Changes in unrealized gains included in earnings related to assets still held at end of period	$ 8,525	$ 8,778