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Securities Available for Sale - Disclosure on Securities Available for Sale with Temporary Impairment (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, fair value $ 432,752 $ 568,290
12 months or longer, fair value 243,044 1,055
Total, fair value 675,796 569,345
Less than 12 months, gross unrealized losses (3,944) (8,405)
12 months or longer, gross unrealized losses (6,172) (14)
Total, gross unrealized losses (10,116) (8,419)
Agency Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, fair value 12,006 14,000
12 months or longer, fair value 6,999 0
Total, fair value 19,005 14,000
Less than 12 months, gross unrealized losses (8) (25)
12 months or longer, gross unrealized losses (1) 0
Total, gross unrealized losses (9) (25)
Mortgage-backed securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, fair value 420,746 553,629
12 months or longer, fair value 235,056 65
Total, fair value 655,802 553,694
Less than 12 months, gross unrealized losses (3,936) (8,377)
12 months or longer, gross unrealized losses (6,159) (3)
Total, gross unrealized losses (10,095) (8,380)
State and municipal obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, fair value   661
12 months or longer, fair value   0
Total, fair value   661
Less than 12 months, gross unrealized losses   (3)
12 months or longer, gross unrealized losses   0
Total, gross unrealized losses   (3)
Corporate obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, fair value 0 0
12 months or longer, fair value 989 990
Total, fair value 989 990
Less than 12 months, gross unrealized losses 0
12 months or longer, gross unrealized losses (12) (11)
Total, gross unrealized losses $ (12) $ (11)