Investment Securities - Schedule of Investment Securities' Gross Unrealized Losses and Fair Value by Investment Category (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	$ 353,265	$ 550,952
Available-for-sale securities, Less than 12 months, Unrealized Loss	4,067	15,091
Available-for-sale securities, 12 months or longer, Fair Value	137,779	4,654
Available-for-sale securities, 12 months or longer, Unrealized Loss	4,620	90
Available-for-sale securities, Total, Fair Value	491,044	555,606
Available-for-sale securities, Total, Unrealized Loss	8,687	15,181
Held-to-maturity, Less than 12 months, Fair Value	13,571	112,642
Held-to-maturity, Less than 12 months, Unrealized Loss	48	1,374
Held-to-maturity, 12 months or longer, Fair Value	6,700	695
Held-to-maturity, 12 months or longer, Unrealized Loss	72	11
Held-to-maturity, Total, Fair Value	20,271	113,337
Held-to-maturity, Total, Unrealized Loss	120	1,385
CMO
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	114,939	160,572
Available-for-sale securities, Less than 12 months, Unrealized Loss	1,188	3,867
Available-for-sale securities, 12 months or longer, Fair Value	43,775	4,654
Available-for-sale securities, 12 months or longer, Unrealized Loss	1,166	90
Available-for-sale securities, Total, Fair Value	158,714	165,226
Available-for-sale securities, Total, Unrealized Loss	2,354	3,957
FNMA MBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	188,153	50,878
Available-for-sale securities, Less than 12 months, Unrealized Loss	2,299	1,330
Available-for-sale securities, 12 months or longer, Fair Value	74,801	0
Available-for-sale securities, 12 months or longer, Unrealized Loss	2,924	0
Available-for-sale securities, Total, Fair Value	262,954	50,878
Available-for-sale securities, Total, Unrealized Loss	5,223	1,330
FHLMC MBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	45,317	300,403
Available-for-sale securities, Less than 12 months, Unrealized Loss	497	9,404
Available-for-sale securities, 12 months or longer, Fair Value	7,892	0
Available-for-sale securities, 12 months or longer, Unrealized Loss	282	0
Available-for-sale securities, Total, Fair Value	53,209	300,403
Available-for-sale securities, Total, Unrealized Loss	779	9,404
GNMA MBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	4,856	16,480
Available-for-sale securities, Less than 12 months, Unrealized Loss	83	427
Available-for-sale securities, 12 months or longer, Fair Value	10,687	0
Available-for-sale securities, 12 months or longer, Unrealized Loss	232	0
Available-for-sale securities, Total, Fair Value	15,543	16,480
Available-for-sale securities, Total, Unrealized Loss	315	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	16	0
Available-for-sale securities, Total, Fair Value	624	623
Available-for-sale securities, Total, Unrealized Loss	16	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, Less than 12 months, Fair Value	13,571	112,642
Held-to-maturity, Less than 12 months, Unrealized Loss	48	1,374
Held-to-maturity, 12 months or longer, Fair Value	6,700	695
Held-to-maturity, 12 months or longer, Unrealized Loss	72	11
Held-to-maturity, Total, Fair Value	20,271	113,337
Held-to-maturity, Total, Unrealized Loss	$ 120	$ 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
-Topic 320
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-Section 50
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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

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