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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Privately Issued Mortgage-Backed Securities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 16	$ 16	$ 16	$ 22
Total gains realized/unrealized:
Settlements				(6)
Ending Balance	16	16	16	16
Other Assets and Other Liabilities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	40,106	17,200	10,740	7,712
Total gains realized/unrealized:
Included in earnings	57,819	62,178	150,632	115,187
Transfers out of Level 3	(46,422)	(55,104)	(109,869)	(98,625)
Ending Balance	51,503	24,274	51,503	24,274
Changes in unrealized gains included in earnings related to assets still held at end of period	$ 44,127	$ 19,350	$ 50,411	$ 24,981