Investment Securities - Schedule of Investment Securities' Gross Unrealized Losses and Fair Value by Investment Category (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	$ 398,776	$ 550,952
Available-for-sale securities, Less than 12 months, Unrealized Loss	3,624	15,091
Available-for-sale securities, 12 months or longer, Fair Value	240,302	4,654
Available-for-sale securities, 12 months or longer, Unrealized Loss	8,044	90
Available-for-sale securities, Total, Fair Value	639,078	555,606
Available-for-sale securities, Total, Unrealized Loss	11,668	15,181
Held-to-maturity securities, Less than 12 months, Fair Value	23,404	112,642
Held-to-maturity securities, Less than 12 months, Unrealized Loss	59	1,374
Held-to-maturity securities, 12 months or longer, Fair Value	5,625	695
Held-to-maturity securities, 12 months or longer, Unrealized Loss	32	11
Held-to-maturity securities, Total, Fair Value	29,029	113,337
Held-to-maturity securities, Total, Unrealized Loss	91	1,385
CMO
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	146,726	160,572
Available-for-sale securities, Less than 12 months, Unrealized Loss	1,820	3,867
Available-for-sale securities, 12 months or longer, Fair Value	77,149	4,654
Available-for-sale securities, 12 months or longer, Unrealized Loss	2,321	90
Available-for-sale securities, Total, Fair Value	223,875	165,226
Available-for-sale securities, Total, Unrealized Loss	4,141	3,957
FNMA MBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	42,514	50,878
Available-for-sale securities, Less than 12 months, Unrealized Loss	269	1,330
Available-for-sale securities, 12 months or longer, Fair Value	21,405	0
Available-for-sale securities, 12 months or longer, Unrealized Loss	655	0
Available-for-sale securities, Total, Fair Value	63,919	50,878
Available-for-sale securities, Total, Unrealized Loss	924	1,330
FHLMC MBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	204,921	300,403
Available-for-sale securities, Less than 12 months, Unrealized Loss	1,479	9,404
Available-for-sale securities, 12 months or longer, Fair Value	126,342	0
Available-for-sale securities, 12 months or longer, Unrealized Loss	4,693	0
Available-for-sale securities, Total, Fair Value	331,263	300,403
Available-for-sale securities, Total, Unrealized Loss	6,172	9,404
GNMA MBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	4,615	16,480
Available-for-sale securities, Less than 12 months, Unrealized Loss	56	427
Available-for-sale securities, 12 months or longer, Fair Value	14,782	0
Available-for-sale securities, 12 months or longer, Unrealized Loss	355	0
Available-for-sale securities, Total, Fair Value	19,397	16,480
Available-for-sale securities, Total, Unrealized Loss	411	427
Other investments
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	0	623
Available-for-sale securities, Less than 12 months, Unrealized Loss	0	3
Available-for-sale securities, 12 months or longer, Fair Value	624	0
Available-for-sale securities, 12 months or longer, Unrealized Loss	20	0
Available-for-sale securities, Total, Fair Value	624	623
Available-for-sale securities, Total, Unrealized Loss	20	3
GSE
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value		21,996
Available-for-sale securities, Less than 12 months, Unrealized Loss		60
Available-for-sale securities, 12 months or longer, Fair Value		0
Available-for-sale securities, 12 months or longer, Unrealized Loss		0
Available-for-sale securities, Total, Fair Value		21,996
Available-for-sale securities, Total, Unrealized Loss		60
State and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity securities, Less than 12 months, Fair Value	23,404	112,642
Held-to-maturity securities, Less than 12 months, Unrealized Loss	59	1,374
Held-to-maturity securities, 12 months or longer, Fair Value	5,625	695
Held-to-maturity securities, 12 months or longer, Unrealized Loss	32	11
Held-to-maturity securities, Total, Fair Value	29,029	113,337
Held-to-maturity securities, Total, Unrealized Loss	$ 91	$ 1,385

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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-Publisher FASB
-Name Accounting Standards Codification
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