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		Jun. 30, 2016		Dec. 31, 2015	[2]
Held-to-Maturity Securities:
Carrying Amount		$ 3,822,561	[1]	$ 5,969,390
Held-to-Maturity Securities:
Due within one year		219
Due from one to five years		676,824
Due from five to ten years		2,867,505
Due after ten years		549,186
Total securities held to maturity		4,093,734	[1]	$ 6,108,529
Available-for-Sale Securities:
Due within one year	[3]	153
Due from one to five years	[3]	658
Due from five to ten years	[3]	0
Due after ten years	[3]	6,793
Total securities available for sale	[3]	7,604
Mortgage-Related Securities
Held-to-Maturity Securities:
Due from one to five years		555,797
Due from five to ten years		2,482,830
Due after ten years		391,180
Carrying Amount		3,429,807
Available-for-Sale Securities:
Due from five to ten years	[3]	$ 0
Held-to-Maturity Securities, Average Yield
Due from one to five years, Average Yield		3.76%
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	[3]	0.00%
U.S. Treasury and GSE Obligations
Held-to-Maturity Securities:
Due from one to five years		$ 66,673
Due from five to ten years		112,693
Carrying Amount		179,366
Available-for-Sale Securities:
Due from five to ten years	[3]	$ 0
Held-to-Maturity Securities, Average Yield
Due from one to five years, Average Yield		4.14%
Due from five to ten years, Average Yield		3.45%
Total securities held to maturity, Average Yield		3.71%
Available-for-Sale Securities, Average Yield
Due from five to ten years, Average Yield	[3]	0.00%
State, county, and municipal
Held-to-Maturity Securities:
Due within one year		$ 219
Due after ten years		73,573
Carrying Amount		73,792
Available-for-Sale Securities:
Due within one year	[3]	149
Due from one to five years	[3]	580
Due from five to ten years	[3]	0
Total securities available for sale	[3]	$ 729
Held-to-Maturity Securities, Average Yield
Due within one year, Average Yield	[4]	2.96%
Due after ten years, Average Yield	[4]	2.89%
Total securities held to maturity, Average Yield	[4]	2.89%
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%
Due from five to ten years, Average Yield	[3],[4]	0.00%
Total securities available for sale, Average Yield	[3],[4]	6.52%
Other Debt Securities
Held-to-Maturity Securities:
Due from five to ten years	[5]	$ 64,313
Due after ten years	[5]	75,283
Carrying Amount	[5]	139,596
Available-for-Sale Securities:
Due from five to ten years	[3],[5]	0
Due after ten years	[3],[5]	9,451
Total securities available for sale	[3],[5]	$ 9,451
Held-to-Maturity Securities, Average Yield
Due from five to ten years, Average Yield		4.75%
Due after ten years, Average Yield		5.20%
Total securities held to maturity, Average Yield		4.99%
Available-for-Sale Securities, Average Yield
Due from five to ten years, Average Yield	[3]	0.00%
Due after ten years, Average Yield	[3]	4.48%
Total securities available for sale, Average Yield	[3]	4.48%

[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 June 30, 2016, the non-credit portion of OTTI recorded in AOCL was $8.7 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