Securities - 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 (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Held-to-Maturity Securities:
Carrying Amount	$ 7,268,244	[1]	$ 7,670,282	[2]
Held-to-Maturity Securities:
Due within one year	0
Due from one to five years	67,544
Due from five to ten years	6,000,178
Due after ten years	1,249,293
Total debt securities held to maturity	7,317,015	[1]	7,445,244	[2]
Available-for-Sale Securities:
Due within one year	126	[3]
Due from one to five years	4,921	[3]
Due from five to ten years	17,416	[3]
Due after ten years	79,377	[3]
Total debt securities available for sale	101,840	[3]
Mortgage-Related Securities
Held-to-Maturity Securities:
Due within one year	0
Due from one to five years	841
Due from five to ten years	3,233,780
Due after ten years	1,045,193
Carrying Amount	4,279,814		4,407,987
Available-for-Sale Securities:
Due from one to five years	4,083	[3]
Due from five to ten years	16,137	[3]
Due after ten years	67,991	[3]
Total debt securities available for sale	88,211	[3]
Held-to-Maturity Securities, Average Yield
Due within one year, Average Yield	0.00%
Due from one to five years, Average Yield	5.89%
Due from five to ten years, Average Yield	3.22%
Due after ten years, Average Yield	3.36%
Total debt securities held to maturity, Average Yield	3.25%
Available-for-Sale Securities, Average Yield
Due from one to five years, Average Yield	6.82%	[3]
Due from five to ten years, Average Yield	3.72%	[3]
Due after ten years, Average Yield	3.59%	[3]
Total debt securities available for sale, Average Yield	3.76%	[3]
Held-to-Maturity Securities:
Total debt securities held to maturity	4,392,753		4,383,662
U.S. Treasury and GSE Obligations
Held-to-Maturity Securities:
Due within one year	0
Due from one to five years	60,190
Due from five to ten years	2,707,852
Due after ten years	12,288
Carrying Amount	2,780,330
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	2.74%
Due after ten years, Average Yield	3.99%
Total debt securities held to maturity, Average Yield	2.78%
State, county, and municipal
Held-to-Maturity Securities:
Due within one year	0
Due from one to five years	967
Due after ten years	58,310
Carrying Amount	59,277
Available-for-Sale Securities:
Due within one year	125	[3]
Due from one to five years	558	[3]
Due from five to ten years	281	[3]
Total debt securities available for sale	964	[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 debt securities held to maturity, Average Yield	2.85%	[4]
Available-for-Sale Securities, Average Yield
Due within one year, Average Yield	6.09%	[3],[4]
Due from one to five years, Average Yield	6.45%	[3],[4]
Due from five to ten years, Average Yield	6.63%	[3],[4]
Total debt securities available for sale, Average Yield	6.46%	[3],[4]
Other Debt Securities
Held-to-Maturity Securities:
Due within one year	0	[5]
Due from five to ten years	47,251	[5]
Due after ten years	101,572	[5]
Carrying Amount	148,823	[5]
Available-for-Sale Securities:
Due after ten years	13,428	[3],[5]
Total debt securities available for sale	$ 13,428	[3],[5]
Held-to-Maturity Securities, Average Yield
Due within one year, Average Yield	0.00%
Due from five to ten years, Average Yield	3.14%
Due after ten years, Average Yield	5.80%
Total debt securities held to maturity, Average Yield	4.96%
Available-for-Sale Securities, Average Yield
Due after ten years, Average Yield	5.67%	[3]
Total debt securities available for sale, Average Yield	5.67%	[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 September 30, 2014, the non-credit portion of OTTI recorded in AOCL was $8.9 million (before taxes).

[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, 2013, the non-credit portion of OTTI recorded in AOCL was $9.2 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] Consists of corporate bonds and capital trust notes. Included in capital trust notes are $247,000 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.