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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Privately Issued Mortgage-Backed Securities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 24	$ 35	$ 28	$ 44
Total gains (losses) realized/unrealized:
Settlements	(1)	(5)	(5)	(14)
Ending Balance	23	30	23	30
Other Assets and Other Liabilities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	10,751	12,425	8,303	7,325
Total gains (losses) realized/unrealized:
Included in earnings	10,284	22,313	38,691	65,824
Transfers out of Level 3	(16,849)	(21,016)	(42,808)	(59,427)
Ending Balance	4,186	13,722	4,186	13,722
Changes in unrealized gains included in earnings related to assets still held at end of period	$ 4,508	$ 12,659	$ 3,663	$ 13,684