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		Sep. 30, 2016		Dec. 31, 2015	[2]
Held-to-Maturity Securities:
Carrying Amount		$ 3,651,925	[1]	$ 5,969,390
Held-to-Maturity Securities:
Due within one year		109
Due from one to five years		830,343
Due from five to ten years		2,550,076
Due after ten years		512,558
Total securities held to maturity		3,893,086	[1]	$ 6,108,529
Available-for-Sale Securities:
Due within one year	[3]	151
Due from one to five years	[3]	648
Due from five to ten years	[3]	7,827
Due after ten years	[3]	6,976
Total securities available for sale	[3]	15,602
Mortgage-Related Securities
Held-to-Maturity Securities:
Due from one to five years		655,716
Due from five to ten years		2,338,989
Due after ten years		356,753
Carrying Amount		3,351,458
Available-for-Sale Securities:
Due from five to ten years	[3]	7,824
Total securities available for sale	[3]	$ 7,824
Held-to-Maturity Securities, Average Yield
Due from one to five years, Average Yield		3.73%
Due from five to ten years, Average Yield		3.09%
Due after ten years, Average Yield		2.97%
Total securities held to maturity, Average Yield		3.20%
Available-for-Sale Securities, Average Yield
Due from five to ten years, Average Yield	[3]	1.90%
Total securities available for sale, Average Yield	[3]	1.90%
U.S. Treasury and GSE Obligations
Held-to-Maturity Securities:
Due from one to five years		$ 66,604
Due from five to ten years		21,923
Carrying Amount		$ 88,527
Held-to-Maturity Securities, Average Yield
Due from one to five years, Average Yield		4.14%
Due from five to ten years, Average Yield		3.52%
Total securities held to maturity, Average Yield		3.99%
State, county, and municipal
Held-to-Maturity Securities:
Due within one year		$ 109
Due after ten years		72,078
Carrying Amount		72,187
Available-for-Sale Securities:
Due within one year	[3]	150
Due from one to five years	[3]	581
Total securities available for sale	[3]	$ 731
Held-to-Maturity Securities, Average Yield
Due within one year, Average Yield	[4]	2.96%
Due after ten years, Average Yield	[4]	2.88%
Total securities held to maturity, Average Yield	[4]	2.88%
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield	[3],[4]	6.39%
Due from one to five years, Average Yield	[3],[4]	6.56%
Total securities available for sale, Average Yield	[3],[4]	6.53%
Other Debt Securities
Held-to-Maturity Securities:
Due from one to five years	[5]	$ 47,970
Due from five to ten years	[5]	16,447
Due after ten years	[5]	75,336
Carrying Amount	[5]	139,753
Available-for-Sale Securities:
Due after ten years	[3],[5]	9,455
Total securities available for sale	[3],[5]	$ 9,455
Held-to-Maturity Securities, Average Yield
Due from one to five years, Average Yield		3.32%
Due from five to ten years, Average Yield		9.05%
Due after ten years, Average Yield		5.25%
Total securities held to maturity, Average Yield		5.03%
Available-for-Sale Securities, Average Yield
Due after ten years, Average Yield	[3]	4.52%
Total securities available for sale, Average Yield	[3]	4.52%

[1] Held-to-maturity securities are reported at a carrying amount equal to amortized cost less the non-credit portion of OTTI recorded in Accumulated Other Comprehensive Loss ("AOCL"). At September 30, 2016, the non-credit portion of OTTI recorded in AOCL was $8.6 million, pre-tax.

[2] 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, 2015, the non-credit portion of OTTI recorded in AOCL was $8.7 million, pre-tax.

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

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

[5] Includes corporate bonds and capital trust notes.