Summary of Amortized Cost of Available-for-Sale Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Available-for-Sale Securities:
Due within one year		$ 153
Due from one to five years		1,233,880
Due from five to ten years		2,213,968
Due after ten years		2,165,519
Total securities available for sale		5,613,520	$ 3,531,427
Mortgage-Related Securities
Available-for-Sale Securities:
Due within one year		0
Due from one to five years		1,093,265
Due from five to ten years		328,455
Due after ten years		1,532,237
Total securities available for sale		$ 2,953,957
Available-for-Sale Securities, Average Yield
Due from one to five years, Average Yield		3.31%
Due from five to ten years, Average Yield		3.41%
Due after ten years, Average Yield		3.20%
Total securities available for sale, Average Yield		3.26%
U.S. Treasury and GSE Obligations
Available-for-Sale Securities:
Due within one year		$ 0
Due from one to five years		32,874
Due from five to ten years		1,148,695
Due after ten years		152,980
Total securities available for sale		$ 1,334,549
Available-for-Sale Securities, Average Yield
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		3.63%
Total securities available for sale, Average Yield		3.44%
State, county, and municipal
Available-for-Sale Securities:
Due within one year		$ 148
Due from one to five years		146
Due from five to ten years		10,981
Due after ten years		57,276
Total securities available for sale		$ 68,551
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]	3.78%
Due after ten years, Average Yield	[1]	2.71%
Total securities available for sale, Average Yield	[1]	2.90%
Other Debt Securities
Available-for-Sale Securities:
Due within one year	[2]	$ 0
Due from one to five years	[2]	92,830
Due from five to ten years	[2]	743,323
Due after ten years	[2]	435,046
Total securities available for sale	[2]	$ 1,271,199
Available-for-Sale Securities, Average Yield
Due from one to five years, Average Yield		3.75%
Due from five to ten years, Average Yield		4.34%
Due after ten years, Average Yield		3.24%
Total securities available for sale, Average Yield		3.92%

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

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