Investment Securities - Summary of Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 2,181,478	$ 2,337,715
Gross Unrealized Holding Gain, Available-for-sale	66,555	33,776
Gross Unrealized Holding Loss, Available-for-sale	(1)	(2,845)
Fair Value, Available-for-sale	$ 2,248,032	$ 2,368,646
Total Percent, Available-for-sale	100.00%	100.00%
Amortized Cost, Held-to-maturity	$ 724,357	$ 850,989
Gross Unrealized Holding Gain, Held-to-maturity	20,094	4,796
Gross Unrealized Holding Loss, Held-to-maturity	(970)	(2,746)
Fair Value, Held-to-maturity	$ 743,481	$ 853,039
Total Percent, Held-to-maturity	100.00%	100.00%
CMO/REMIC - Residential [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 388,881	$ 380,166
Gross Unrealized Holding Gain, Available-for-sale	9,015	4,689
Gross Unrealized Holding Loss, Available-for-sale	0	(1,074)
Fair Value, Available-for-sale	$ 397,896	$ 383,781
Total Percent, Available-for-sale	17.70%	16.20%
Amortized Cost, Held-to-maturity	$ 974	$ 1,284
Gross Unrealized Holding Gain, Held-to-maturity	501	569
Gross Unrealized Holding Loss, Held-to-maturity	0	0
Fair Value, Held-to-maturity	$ 1,475	$ 1,853
Total Percent, Held-to-maturity	0.13%	0.15%
Government Agency/GSE [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 4,750	$ 5,752
Gross Unrealized Holding Gain, Available-for-sale	13	0
Gross Unrealized Holding Loss, Available-for-sale	0	(7)
Fair Value, Available-for-sale	$ 4,763	$ 5,745
Total Percent, Available-for-sale	0.21%	0.24%
Amortized Cost, Held-to-maturity	$ 209,301	$ 293,338
Gross Unrealized Holding Gain, Held-to-maturity	6,336	1,176
Gross Unrealized Holding Loss, Held-to-maturity	0	(734)
Fair Value, Held-to-maturity	$ 215,637	$ 293,780
Total Percent, Held-to-maturity	28.90%	34.47%
Residential Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 1,663,091	$ 1,788,857
Gross Unrealized Holding Gain, Available-for-sale	54,359	26,001
Gross Unrealized Holding Loss, Available-for-sale	0	(1,761)
Fair Value, Available-for-sale	$ 1,717,450	$ 1,813,097
Total Percent, Available-for-sale	76.40%	76.55%
Amortized Cost, Held-to-maturity	$ 215,762	$ 232,053
Gross Unrealized Holding Gain, Held-to-maturity	6,274	0
Gross Unrealized Holding Loss, Held-to-maturity	0	(1,293)
Fair Value, Held-to-maturity	$ 222,036	$ 230,760
Total Percent, Held-to-maturity	29.79%	27.27%
Municipal Bonds [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 119,756	$ 157,940
Gross Unrealized Holding Gain, Available-for-sale	2,893	3,036
Gross Unrealized Holding Loss, Available-for-sale	(1)	(3)
Fair Value, Available-for-sale	$ 122,648	$ 160,973
Total Percent, Available-for-sale	5.46%	6.80%
Amortized Cost, Held-to-maturity	$ 298,320	$ 324,314
Gross Unrealized Holding Gain, Held-to-maturity	6,983	3,051
Gross Unrealized Holding Loss, Held-to-maturity	(970)	(719)
Fair Value, Held-to-maturity	$ 304,333	$ 326,646
Total Percent, Held-to-maturity	41.18%	38.11%
Other Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 5,000	$ 5,000
Gross Unrealized Holding Gain, Available-for-sale	275	50
Gross Unrealized Holding Loss, Available-for-sale	0	0
Fair Value, Available-for-sale	$ 5,275	$ 5,050
Total Percent, Available-for-sale	0.23%	0.21%