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	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Privately Issued Mortgage-Backed Securities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 22	$ 28	$ 44
Total gains realized/unrealized:
Settlements	(6)	(6)	(16)
Ending Balance	16	22	28
Other Assets and Other Liabilities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	7,712	8,303	7,325
Total gains realized/unrealized:
Included in earnings	129,398	58,740	77,832
Transfers out of Level 3	(126,370)	(59,331)	(76,854)
Ending Balance	10,740	7,712	8,303
Changes in unrealized gains included in earnings related to assets still held at end of period	$ 11,146	$ 7,386	$ 7,978