Summary of Carrying Amount and Estimated Fair Value of Held-to-Maturity Debt Securities and Amortized Cost and Estimated Fair Value of Available-for-Sale Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Mar. 31, 2017	Dec. 31, 2016	[1]
Held-to-Maturity Securities:
Carrying Amount		$ 3,642,104	$ 3,712,776
Held-to-Maturity Securities:
Due within one year		261,164
Due from one to five years		788,750
Due from five to ten years		2,258,597
Due after ten years		441,538
Total securities held to maturity		3,750,049	$ 3,813,959
Available-for-Sale Securities:
Due within one year	[2]	153
Due from one to five years	[2]	478
Due from five to ten years	[2]	7,303
Due after ten years	[2]	7,417
Total securities available for sale	[2]	15,351
Mortgage-Related Securities
Held-to-Maturity Securities:
Due from one to five years		699,621
Due from five to ten years		2,133,551
Due after ten years		309,021
Carrying Amount		3,142,193
Available-for-Sale Securities:
Due from five to ten years	[2]	7,748
Total securities available for sale	[2]	$ 7,748
Held-to-Maturity Securities, Average Yield
Due from one to five years, Average Yield		3.57%
Due from five to ten years, Average Yield		3.11%
Due after ten years, Average Yield		2.98%
Total securities held to maturity, Average Yield		3.20%
Available-for-Sale Securities, Average Yield
Due from five to ten years, Average Yield	[2]	1.90%
Total securities available for sale, Average Yield	[2]	1.90%
U.S. Treasury and GSE Obligations
Held-to-Maturity Securities:
Due within one year		$ 259,671
Due from one to five years		6,950
Due from five to ten years		21,924
Carrying Amount		$ 288,545
Held-to-Maturity Securities, Average Yield
Due within one year, Average Yield		1.42%
Due from one to five years, Average Yield		3.84%
Due from five to ten years, Average Yield		3.52%
Total securities held to maturity, Average Yield		1.64%
State, county, and municipal
Held-to-Maturity Securities:
Due after ten years		$ 71,297
Carrying Amount		71,297
Available-for-Sale Securities:
Due within one year	[2]	149
Due from one to five years	[2]	435
Total securities available for sale	[2]	$ 584
Held-to-Maturity Securities, Average Yield
Due after ten years, Average Yield	[3]	2.88%
Total securities held to maturity, Average Yield	[3]	2.88%
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield	[2],[3]	6.47%
Due from one to five years, Average Yield	[2],[3]	6.59%
Total securities available for sale, Average Yield	[2],[3]	6.56%
Other Debt Securities
Held-to-Maturity Securities:
Due from one to five years	[4]	$ 48,158
Due from five to ten years	[4]	26,179
Due after ten years	[4]	65,732
Carrying Amount	[4]	140,069
Available-for-Sale Securities:
Due after ten years	[2],[4]	9,463
Total securities available for sale	[2],[4]	$ 9,463
Held-to-Maturity Securities, Average Yield
Due from one to five years, Average Yield		3.40%
Due from five to ten years, Average Yield		9.06%
Due after ten years, Average Yield		4.78%
Total securities held to maturity, Average Yield		5.10%
Available-for-Sale Securities, Average Yield
Due after ten years, Average Yield	[2]	4.70%
Total securities available for sale, Average Yield	[2]	4.70%

[1] Held-to-maturity securities are reported at a carrying amount equal to amortized cost less the non-credit portion of OTTI recorded in AOCL. At December 31, 2016, the non-credit portion of OTTI recorded in AOCL was $8.6 million (before taxes).

[2] As equity securities have no contractual maturity, they have been excluded from this table.

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

[4] Includes corporate bonds and capital trust notes.