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, 2020	Dec. 31, 2019	Dec. 31, 2018
Privately Issued Mortgage-Backed Securities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 16	$ 22	$ 28
Total gains realized/unrealized:
Settlements		(6)	(6)
Ending Balance	16	16	22
Other Assets and Other Liabilities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	10,740	7,712	8,303
Total gains realized/unrealized:
Included in earnings	194,469	129,398	58,740
Transfers out of Level 3	(161,975)	(126,370)	(59,331)
Ending Balance	43,234	10,740	7,712
Changes in unrealized gains included in earnings related to assets still held at end of period	$ 42,597	$ 11,146	$ 7,386