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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Privately Issued Mortgage-Backed Securities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 27	$ 41	$ 28	$ 44
Total gains (losses) realized/unrealized:
Settlements	(3)	(6)	(4)	(9)
Ending Balance	24	35	24	35
Other Assets and Other Liabilities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	8,760	16,202	8,303	7,325
Total gains (losses) realized/unrealized:
Included in earnings	20,277	19,571	28,407	43,511
Transfers out of Level 3	(18,286)	(23,348)	(25,959)	(38,411)
Ending Balance	10,751	12,425	10,751	12,425
Changes in unrealized gains included in earnings related to assets still held at end of period	$ 10,686	$ 10,892	$ 10,686	$ 12,372