Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt securities available for sale:
Due in one year or less	$ 1,269,955
Due after one year through five years	361,249
Due after five years through ten years	101,971
Due after ten years	30,029
Total available for sale (amortized cost)	1,763,204
Mortgage-backed securities available for sale	7,686,771
Investment securities available for sale, amortized cost	9,449,975	$ 10,938,796
Debt securities held to maturity:
Due in one year or less	448,774
Due after one year through five years	5,177
Due after ten years	3,965
Total available for held to maturity (amortized cost)	457,916
Mortgage-backed securities held to maturity	2,960,803
Amortized cost for held to maturity	3,418,719	3,353,213
Debt securities available for sale:
Due in one year or less	1,261,175
Due after one year through five years	354,788
Due after five years through ten years	101,055
Due after ten years	24,778
Total available for sale (fair value)	1,741,796
Mortgage-backed securities available for sale	7,412,719
Total	9,154,515	10,896,284
Debt securities held to maturity:
Due in one year or less	448,366
Due after one year through five years	5,187
Due after ten years	3,965
Total available for held to maturity (fair value)	457,518
Mortgage-backed securities held to maturity	2,855,752
Total	$ 3,313,270	$ 3,341,762