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, 2015		Dec. 31, 2014
Held-to-Maturity Securities:
Carrying Amount		$ 6,597,285	[1]	$ 6,922,667	[2]
Held-to-Maturity Securities:
Due within one year		0
Due from one to five years		67,793
Due from five to ten years		6,101,352
Due after ten years		643,629
Total securities held to maturity		6,812,774	[1]	7,085,971	[2]
Available-for-Sale Securities:
Due within one year	[3]	126
Due from one to five years	[3]	3,244
Due from five to ten years	[3]	816
Due after ten years	[3]	17,797
Total securities available for sale	[3]	21,983
Mortgage-Related Securities
Held-to-Maturity Securities:
Due within one year		0
Due from one to five years		2,749
Due from five to ten years		3,268,114
Due after ten years		485,473
Carrying Amount		3,756,336		4,079,034
Available-for-Sale Securities:
Due from one to five years	[3]	2,549
Due from five to ten years	[3]	618
Due after ten years	[3]	10,036
Total securities available for sale	[3]	$ 13,203
Held-to-Maturity Securities, Average Yield
Due within one year, Average Yield		0.00%
Due from one to five years, Average Yield		3.39%
Due from five to ten years, Average Yield		3.26%
Due after ten years, Average Yield		3.05%
Total securities held to maturity, Average Yield		3.23%
Available-for-Sale Securities, Average Yield
Due from one to five years, Average Yield	[3]	6.57%
Due from five to ten years, Average Yield	[3]	4.38%
Due after ten years, Average Yield	[3]	4.70%
Total securities available for sale, Average Yield	[3]	5.05%
Held-to-Maturity Securities:
Total securities held to maturity		$ 3,950,417		$ 4,245,974
U.S. Treasury and GSE Obligations
Held-to-Maturity Securities:
Due within one year		0
Due from one to five years		59,927
Due from five to ten years		2,564,673
Carrying Amount		$ 2,624,600
Held-to-Maturity Securities, Average Yield
Due within one year, Average Yield		0.00%
Due from one to five years, Average Yield		4.17%
Due from five to ten years, Average Yield		2.70%
Total securities held to maturity, Average Yield		2.73%
State, county, and municipal
Held-to-Maturity Securities:
Due within one year		$ 0
Due from one to five years		543
Due after ten years		76,597
Carrying Amount		77,140
Available-for-Sale Securities:
Due within one year	[3]	125
Due from one to five years	[3]	582
Due from five to ten years	[3]	141
Total securities available for sale	[3]	$ 848
Held-to-Maturity Securities, Average Yield
Due within one year, Average Yield	[4]	0.00%
Due from one to five years, Average Yield	[4]	2.96%
Due after ten years, Average Yield	[4]	2.91%
Total securities held to maturity, Average Yield	[4]	2.91%
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield	[3],[4]	6.45%
Due from one to five years, Average Yield	[3],[4]	6.49%
Due from five to ten years, Average Yield	[3],[4]	6.66%
Total securities available for sale, Average Yield	[3],[4]	6.51%
Other Debt Securities
Held-to-Maturity Securities:
Due within one year	[5]	$ 0
Due from five to ten years	[5]	64,009
Due after ten years	[5]	75,200
Carrying Amount	[5]	139,209
Available-for-Sale Securities:
Due after ten years	[3],[5]	9,441
Total securities available for sale	[3],[5]	$ 9,441
Held-to-Maturity Securities, Average Yield
Due within one year, Average Yield		0.00%
Due from five to ten years, Average Yield		4.68%
Due after ten years, Average Yield		5.06%
Total securities held to maturity, Average Yield		4.89%
Available-for-Sale Securities, Average Yield
Due after ten years, Average Yield	[3]	4.26%
Total securities available for sale, Average Yield	[3]	4.26%

[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 September 30, 2015, the non-credit portion of OTTI recorded in AOCL was $8.7 million (before 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, 2014, the non-credit portion of OTTI recorded in AOCL was $8.8 million (before taxes).

[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. Included in capital trust notes are $62,000 of pooled trust preferred securities held to maturity, all of which are due after ten years. The remaining capital trust notes consist of single-issue trust preferred securities.