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, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Held-to-Maturity Securities:
Carrying Amount	$ 6,784,883	[1]	$ 6,922,667	[2]
Held-to-Maturity Securities:
Due within one year	0
Due from one to five years	66,125
Due from five to ten years	5,932,713
Due after ten years	1,029,363
Total securities held to maturity	7,028,201	[1]	7,085,971	[2]
Available-for-Sale Securities:
Due within one year	129	[3]
Due from one to five years	4,111	[3]
Due from five to ten years	3,505	[3]
Due after ten years	22,970	[3]
Total securities available for sale	30,715	[3]
Mortgage-Related Securities
Held-to-Maturity Securities:
Due within one year	0
Due from five to ten years	3,119,709
Due after ten years	835,430
Carrying Amount	3,955,139		4,079,034
Available-for-Sale Securities:
Due within one year	1	[3]
Due from one to five years	3,327	[3]
Due from five to ten years	3,112	[3]
Due after ten years	10,960	[3]
Total securities available for sale	17,400	[3]
Held-to-Maturity Securities, Average Yield
Due within one year, Average Yield	0.00%
Due from five to ten years, Average Yield	3.23%
Due after ten years, Average Yield	3.27%
Total securities held to maturity, Average Yield	3.24%
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield	4.41%	[3]
Due from one to five years, Average Yield	6.72%	[3]
Due from five to ten years, Average Yield	3.78%	[3]
Due after ten years, Average Yield	4.67%	[3]
Total securities available for sale, Average Yield	4.90%	[3]
Held-to-Maturity Securities:
Total securities held to maturity	4,171,837		4,245,974
U.S. Treasury and GSE Obligations
Held-to-Maturity Securities:
Due within one year	0
Due from one to five years	60,060
Due from five to ten years	2,572,267
Carrying Amount	2,632,327
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	756
Due after ten years	57,620
Carrying Amount	58,376
Available-for-Sale Securities:
Due within one year	124	[3]
Due from one to five years	579	[3]
Due from five to ten years	140	[3]
Total securities available for sale	843	[3]
Held-to-Maturity Securities, Average Yield
Due within one year, Average Yield	0.00%	[4]
Due from one to five years, Average Yield	2.96%	[4]
Due after ten years, Average Yield	2.85%	[4]
Total securities held to maturity, Average Yield	2.85%	[4]
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield	6.45%	[3],[4]
Due from one to five years, Average Yield	6.49%	[3],[4]
Due from five to ten years, Average Yield	6.66%	[3],[4]
Total securities available for sale, Average Yield	6.51%	[3],[4]
Other Debt Securities
Held-to-Maturity Securities:
Due within one year	0	[5]
Due from five to ten years	47,425	[5]
Due after ten years	91,616	[5]
Carrying Amount	139,041	[5]
Available-for-Sale Securities:
Due after ten years	13,434	[3],[5]
Total securities available for sale	$ 13,434	[3],[5]
Held-to-Maturity Securities, Average Yield
Due within one year, Average Yield	0.00%
Due from five to ten years, Average Yield	3.15%
Due after ten years, Average Yield	5.76%
Total securities held to maturity, Average Yield	4.87%
Available-for-Sale Securities, Average Yield
Due after ten years, Average Yield	5.70%	[3]
Total securities available for sale, Average Yield	5.70%	[3]

[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 March 31, 2015, the non-credit portion of OTTI recorded in AOCL was $8.8 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 $183,000 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.