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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Privately Issued Mortgage-Backed Securities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 16	$ 27	$ 22	$ 28
Total gains realized/unrealized:
Settlements		(3)	(6)	(4)
Ending Balance	16	24	16	24
Other Assets and Other Liabilities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	7,433	8,760	7,712	8,303
Total gains realized/unrealized:
Included in earnings	36,463	20,277	53,009	28,407
Transfers out of Level 3	(26,696)	(18,286)	(43,521)	(25,959)
Ending Balance	17,200	10,751	17,200	10,751
Changes in unrealized gains included in earnings related to assets still held at end of period	$ 17,464	$ 10,686	$ 17,372	$ 10,686