Investment Securities - Summary of Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 2,155,330	$ 2,255,874
Gross Unrealized Holding Gain, Available-for-sale	25,555	23,264
Gross Unrealized Holding Loss, Available-for-sale	(5,237)	(8,672)
Fair Value, Available-for-sale	$ 2,175,648	$ 2,270,466
Total Percent, Available-for-sale	100.00%	100.00%
Amortized Cost, Held-to-maturity	$ 848,382	$ 911,676
Gross Unrealized Holding Gain, Held-to-maturity	5,212	1,138
Gross Unrealized Holding Loss, Held-to-maturity	(11,544)	(15,440)
Fair Value, Held-to-maturity	$ 842,050	$ 897,374
Total Percent, Held-to-maturity	100.00%	100.00%
CMO/REMIC - Residential [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 291,984	$ 345,313
Gross Unrealized Holding Gain, Available-for-sale	2,665	3,361
Gross Unrealized Holding Loss, Available-for-sale	(867)	(1,485)
Fair Value, Available-for-sale	$ 293,782	$ 347,189
Total Percent, Available-for-sale	13.50%	15.29%
Amortized Cost, Held-to-maturity	$ 229,885	$ 244,419
Gross Unrealized Holding Gain, Held-to-maturity	0	0
Gross Unrealized Holding Loss, Held-to-maturity	(6,865)	(6,808)
Fair Value, Held-to-maturity	$ 223,020	$ 237,611
Total Percent, Held-to-maturity	27.09%	26.81%
Government Agency/GSE [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale		$ 2,750
Gross Unrealized Holding Gain, Available-for-sale		2
Gross Unrealized Holding Loss, Available-for-sale		0
Fair Value, Available-for-sale		$ 2,752
Total Percent, Available-for-sale		0.12%
Amortized Cost, Held-to-maturity	$ 164,886	$ 182,648
Gross Unrealized Holding Gain, Held-to-maturity	1,532	362
Gross Unrealized Holding Loss, Held-to-maturity	(1,489)	(1,972)
Fair Value, Held-to-maturity	$ 164,929	$ 181,038
Total Percent, Held-to-maturity	19.44%	20.03%
Residential Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 1,799,972	$ 1,822,168
Gross Unrealized Holding Gain, Available-for-sale	22,164	18,812
Gross Unrealized Holding Loss, Available-for-sale	(4,102)	(6,232)
Fair Value, Available-for-sale	$ 1,818,034	$ 1,834,748
Total Percent, Available-for-sale	83.57%	80.81%
Amortized Cost, Held-to-maturity	$ 178,246	$ 193,699
Gross Unrealized Holding Gain, Held-to-maturity	880	0
Gross Unrealized Holding Loss, Held-to-maturity	(61)	(1,892)
Fair Value, Held-to-maturity	$ 179,065	$ 191,807
Total Percent, Held-to-maturity	21.01%	21.25%
Municipal Bonds [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 62,657	$ 80,137
Gross Unrealized Holding Gain, Available-for-sale	726	889
Gross Unrealized Holding Loss, Available-for-sale	(268)	(955)
Fair Value, Available-for-sale	$ 63,115	$ 80,071
Total Percent, Available-for-sale	2.90%	3.53%
Amortized Cost, Held-to-maturity	$ 275,365	$ 290,910
Gross Unrealized Holding Gain, Held-to-maturity	2,800	776
Gross Unrealized Holding Loss, Held-to-maturity	(3,129)	(4,768)
Fair Value, Held-to-maturity	$ 275,036	$ 286,918
Total Percent, Held-to-maturity	32.46%	31.91%
Other Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 717	$ 5,506
Gross Unrealized Holding Gain, Available-for-sale	0	200
Gross Unrealized Holding Loss, Available-for-sale	0	0
Fair Value, Available-for-sale	$ 717	$ 5,706
Total Percent, Available-for-sale	0.03%	0.25%