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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Privately Issued Mortgage-Backed Securities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 16	$ 24	$ 22	$ 28
Total gains realized/unrealized:
Settlements		(1)	(6)	(5)
Ending Balance	16	23	16	23
Other Assets and Other Liabilities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	17,200	10,751	7,712	8,303
Total gains realized/unrealized:
Included in earnings	62,178	10,284	115,187	38,691
Transfers out of Level 3	(55,104)	(16,849)	(98,625)	(42,808)
Ending Balance	24,274	4,186	24,274	4,186
Changes in unrealized gains included in earnings related to assets still held at end of period	$ 19,350	$ 4,508	$ 24,981	$ 3,663