Summary of Amortized Cost of Available-for-Sale Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018
Available-for-Sale Securities:
Due within one year
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield
Available-for-Sale Securities:
Due within one year		$ 44,042
Due from one to five years		977,601
Due from five to ten years		2,495,243
Due after ten years		2,221,260
Total securities available for sale		5,738,146	$ 5,613,520
Mortgage-related Securities
Available-for-Sale Securities:
Due from one to five years		768,275
Due from five to ten years		343,552
Due after ten years		1,611,223
Total securities available for sale		$ 2,723,050
Available-for-Sale Securities, Average Yield
Due from one to five years, Average Yield		3.30%
Due from five to ten years, Average Yield		3.41%
Due after ten years, Average Yield		3.22%
Total securities available for sale, Average Yield		3.26%
U.S. Treasury and GSE Obligations
Available-for-Sale Securities:
Due within one year		$ 29,840
Due from one to five years		32,874
Due from five to ten years		1,405,546
Due after ten years		163,850
Total securities available for sale		$ 1,632,110
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield		2.45%
Due from one to five years, Average Yield		3.48%
Due from five to ten years, Average Yield		3.41%
Due after ten years, Average Yield		3.66%
Total securities available for sale, Average Yield		3.42%
State, county, and municipal
Available-for-Sale Securities:
Due within one year		$ 149
Due from one to five years		147
Due from five to ten years		10,966
Due after ten years		16,121
Total securities available for sale		$ 27,383
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield	[1]	6.59%
Due from one to five years, Average Yield	[1]	6.66%
Due from five to ten years, Average Yield	[1]	3.79%
Due after ten years, Average Yield	[1]	3.22%
Total securities available for sale, Average Yield	[1]	3.49%
Other Debt Securities
Available-for-Sale Securities:
Due within one year	[2]	$ 13,963
Due from one to five years	[2]	154,338
Due from five to ten years	[2]	711,760
Due after ten years	[2]	413,609
Total securities available for sale	[2]	$ 1,293,670
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield		3.79%
Due from one to five years, Average Yield		3.62%
Due from five to ten years, Average Yield		4.38%
Due after ten years, Average Yield		3.22%
Total securities available for sale, Average Yield		3.91%

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

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