Summary of Amortized Cost of Available-for-Sale Securities by Contractual Maturity (Detail) $ in Thousands	Mar. 31, 2020 USD ($)
Available-for-Sale Securities:
Due within one year	$ 136,509
Due from one to five years	816,790
Due from five to ten years	1,457,756
Due after ten years	3,044,190
Total securities available for sale	5,455,245
Mortgage-related Securities
Available-for-Sale Securities:
Due within one year	60,163
Due from one to five years	587,661
Due from five to ten years	255,806
Due after ten years	2,386,837
Total securities available for sale	$ 3,290,467
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield	3.37%
Due from one to five years, Average Yield	3.01%
Due from five to ten years, Average Yield	3.30%
Due after ten years, Average Yield	2.79%
Total securities available for sale, Average Yield	2.88%
U.S. Treasury and GSE Obligations
Available-for-Sale Securities:
Due within one year	$ 61,812
Due from one to five years	32,874
Due from five to ten years	463,621
Due after ten years	221,000
Total securities available for sale	$ 779,307
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield	0.94%
Due from one to five years, Average Yield	3.48%
Due from five to ten years, Average Yield	3.24%
Due after ten years, Average Yield	2.53%
Total securities available for sale, Average Yield	2.87%
State, county, and municipal
Available-for-Sale Securities:
Due within one year	$ 149
Due from five to ten years	20,711
Due after ten years	5,885
Total securities available for sale	$ 26,745
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield	6.66%	[1]
Due from five to ten years, Average Yield	3.49%	[1]
Due after ten years, Average Yield	3.33%	[1]
Total securities available for sale, Average Yield	3.47%	[1]
Other Debt Securities
Available-for-Sale Securities:
Due within one year	$ 13,993	[2]
Due from one to five years	179,651	[2]
Due from five to ten years	749,026	[2]
Due after ten years	390,150	[2]
Total securities available for sale	$ 1,332,820	[2]
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield	3.79%
Due from one to five years, Average Yield	3.18%
Due from five to ten years, Average Yield	2.97%
Due after ten years, Average Yield	1.93%
Total securities available for sale, Average Yield	2.70%

[1] Not presented on a tax-equivalent basis.

[2] Includes corporate bonds, capital trust notes, and asset-backed securities.