Summary of Remaining Contractual Principal Maturities and Fully Taxable Equivalent Yields on Debt Securities Classified as Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	$ 11,338,657
Weighted- Average Yield	1.71%
One Year or Less - Carrying Value	178,088
One Year or Less - Weighted- Average Yield	1.47%
After One Year to Five Years - Carrying Value	2,611,303
After One Year to Five Years - Weighted- Average Yield	1.51%
After Five Years to Ten Years - Carrying Value	2,092,210
After Five Years to Ten Years - Weighted- Average Yield	2.17%
After Ten Years - Carrying Value	6,457,056
After Ten Years - Weighted- Average Yield	1.66%
U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	25,247
Weighted- Average Yield	2.39%
One Year or Less - Carrying Value	25,247
One Year or Less - Weighted- Average Yield	2.39%
After One Year to Five Years - Carrying Value	0
After One Year to Five Years - Weighted- Average Yield	0.00%
After Five Years to Ten Years - Carrying Value	0
After Five Years to Ten Years - Weighted- Average Yield	0.00%
After Ten Years - Carrying Value	0
After Ten Years - Weighted- Average Yield	0.00%
U.S. agency debentures
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	3,447,628
Weighted- Average Yield	1.52%
One Year or Less - Carrying Value	151,916
One Year or Less - Weighted- Average Yield	1.30%
After One Year to Five Years - Carrying Value	2,593,853
After One Year to Five Years - Weighted- Average Yield	1.48%
After Five Years to Ten Years - Carrying Value	701,859
After Five Years to Ten Years - Weighted- Average Yield	1.72%
After Ten Years - Carrying Value	0
After Ten Years - Weighted- Average Yield	0.00%
Residential mortgage-backed securities | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	1,473,433
Weighted- Average Yield	2.33%
One Year or Less - Carrying Value	0
One Year or Less - Weighted- Average Yield	0.00%
After One Year to Five Years - Carrying Value	0
After One Year to Five Years - Weighted- Average Yield	0.00%
After Five Years to Ten Years - Carrying Value	1,342,062
After Five Years to Ten Years - Weighted- Average Yield	2.26%
After Ten Years - Carrying Value	131,371
After Ten Years - Weighted- Average Yield	3.00%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	4,103,974
Weighted- Average Yield	1.98%
One Year or Less - Carrying Value	0
One Year or Less - Weighted- Average Yield	0.00%
After One Year to Five Years - Carrying Value	0
After One Year to Five Years - Weighted- Average Yield	0.00%
After Five Years to Ten Years - Carrying Value	0
After Five Years to Ten Years - Weighted- Average Yield	0.00%
After Ten Years - Carrying Value	4,103,974
After Ten Years - Weighted- Average Yield	1.98%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	1,772,748
Weighted- Average Yield	0.70%
One Year or Less - Carrying Value	0
One Year or Less - Weighted- Average Yield	0.00%
After One Year to Five Years - Carrying Value	0
After One Year to Five Years - Weighted- Average Yield	0.00%
After Five Years to Ten Years - Carrying Value	0
After Five Years to Ten Years - Weighted- Average Yield	0.00%
After Ten Years - Carrying Value	1,772,748
After Ten Years - Weighted- Average Yield	0.70%
Agency-issued commercial mortgage-backed securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	422,098
Weighted- Average Yield	1.82%
One Year or Less - Carrying Value	0
One Year or Less - Weighted- Average Yield	0.00%
After One Year to Five Years - Carrying Value	0
After One Year to Five Years - Weighted- Average Yield	0.00%
After Five Years to Ten Years - Carrying Value	0
After Five Years to Ten Years - Weighted- Average Yield	0.00%
After Ten Years - Carrying Value	422,098
After Ten Years - Weighted- Average Yield	1.82%
Municipal bonds and notes
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	93,529
Weighted- Average Yield	5.99%
One Year or Less - Carrying Value	925
One Year or Less - Weighted- Average Yield	5.20%
After One Year to Five Years - Carrying Value	17,450
After One Year to Five Years - Weighted- Average Yield	5.62%
After Five Years to Ten Years - Carrying Value	48,289
After Five Years to Ten Years - Weighted- Average Yield	5.99%
After Ten Years - Carrying Value	$ 26,865
After Ten Years - Weighted- Average Yield	6.26%