Investment Securities - Summary of Continuous Unrealized Loss Position of Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Fair Value, Less Than 12 Months, Available-for-sale	$ 409,154	$ 734,993
Gross Unrealized Holding Losses, Less Than 12 Months, Available-for-sale	(4,832)	(8,671)
Fair Value, 12 Months or Longer, Available-for-sale	39,450	5,981
Gross Unrealized Holding Losses, 12 Months or Longer, Available-for-sale	(405)	(1)
Fair Value, Available-for-sale	448,604	740,974
Gross Unrealized Holding Losses, Available-for-sale	(5,237)	(8,672)
Fair Value, Less Than 12 Months, Held-to-maturity	343,270	652,076
Gross Unrealized Holding Losses, Less Than 12 Months, Held-to-maturity	(8,242)	(14,383)
Fair Value, 12 Months or Longer, Held-to-maturity	87,600	36,971
Gross Unrealized Holding Losses, 12 Months or Longer, Held-to-maturity	(3,302)	(1,057)
Fair Value, Held-to-maturity	430,870	689,047
Gross Unrealized Holding Losses, Held-to-maturity	(11,544)	(15,440)
CMO/REMIC - Residential [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than 12 Months, Available-for-sale	42,017	128,595
Gross Unrealized Holding Losses, Less Than 12 Months, Available-for-sale	(463)	(1,485)
Fair Value, 12 Months or Longer, Available-for-sale	33,454	0
Gross Unrealized Holding Losses, 12 Months or Longer, Available-for-sale	(404)	0
Fair Value, Available-for-sale	75,471	128,595
Gross Unrealized Holding Losses, Available-for-sale	(867)	(1,485)
Fair Value, Less Than 12 Months, Held-to-maturity	173,752	237,611
Gross Unrealized Holding Losses, Less Than 12 Months, Held-to-maturity	(5,200)	(6,808)
Fair Value, 12 Months or Longer, Held-to-maturity	49,268	0
Gross Unrealized Holding Losses, 12 Months or Longer, Held-to-maturity	(1,665)	0
Fair Value, Held-to-maturity	223,020	237,611
Gross Unrealized Holding Losses, Held-to-maturity	(6,865)	(6,808)
Government Agency/GSE [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than 12 Months, Held-to-maturity	39,929	76,854
Gross Unrealized Holding Losses, Less Than 12 Months, Held-to-maturity	(1,305)	(1,972)
Fair Value, 12 Months or Longer, Held-to-maturity	5,411	0
Gross Unrealized Holding Losses, 12 Months or Longer, Held-to-maturity	(184)	0
Fair Value, Held-to-maturity	45,340	76,854
Gross Unrealized Holding Losses, Held-to-maturity	(1,489)	(1,972)
Residential Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than 12 Months, Available-for-sale	352,129	583,143
Gross Unrealized Holding Losses, Less Than 12 Months, Available-for-sale	(4,102)	(6,232)
Fair Value, 12 Months or Longer, Available-for-sale	0	0
Gross Unrealized Holding Losses, 12 Months or Longer, Available-for-sale	0	0
Fair Value, Available-for-sale	352,129	583,143
Gross Unrealized Holding Losses, Available-for-sale	(4,102)	(6,232)
Fair Value, Less Than 12 Months, Held-to-maturity	62,677	191,807
Gross Unrealized Holding Losses, Less Than 12 Months, Held-to-maturity	(61)	(1,892)
Fair Value, 12 Months or Longer, Held-to-maturity	0	0
Gross Unrealized Holding Losses, 12 Months or Longer, Held-to-maturity	0	0
Fair Value, Held-to-maturity	62,677	191,807
Gross Unrealized Holding Losses, Held-to-maturity	(61)	(1,892)
Municipal Bonds [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than 12 Months, Available-for-sale	15,008	23,255
Gross Unrealized Holding Losses, Less Than 12 Months, Available-for-sale	(267)	(954)
Fair Value, 12 Months or Longer, Available-for-sale	5,996	5,981
Gross Unrealized Holding Losses, 12 Months or Longer, Available-for-sale	(1)	(1)
Fair Value, Available-for-sale	21,004	29,236
Gross Unrealized Holding Losses, Available-for-sale	(268)	(955)
Fair Value, Less Than 12 Months, Held-to-maturity	66,912	145,804
Gross Unrealized Holding Losses, Less Than 12 Months, Held-to-maturity	(1,676)	(3,711)
Fair Value, 12 Months or Longer, Held-to-maturity	32,921	36,971
Gross Unrealized Holding Losses, 12 Months or Longer, Held-to-maturity	(1,453)	(1,057)
Fair Value, Held-to-maturity	99,833	182,775
Gross Unrealized Holding Losses, Held-to-maturity	$ (3,129)	$ (4,768)