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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, 2011
Investments Classified by Contractual Maturity Date [Line Items]  
Carrying Value $ 10,531,511
Weighted- Average Yield 1.85%
One Year or Less - Carrying Value 38,742
One Year or Less - Weighted- Average Yield 2.24%
After One Year to Five Years - Carrying Value 2,697,387
After One Year to Five Years - Weighted- Average Yield 1.58%
After Five Years to Ten Years - Carrying Value 1,661,779
After Five Years to Ten Years - Weighted- Average Yield 2.57%
After Ten Years - Carrying Value 6,133,603
After Ten Years - Weighted- Average Yield 1.78%
Agency-issued commercial mortgage-backed securities
  
Investments Classified by Contractual Maturity Date [Line Items]  
Carrying Value 178,693
Weighted- Average Yield 2.11%
After Ten Years - Carrying Value 178,693
After Ten Years - Weighted- Average Yield 2.11%
U.S. treasury securities
  
Investments Classified by Contractual Maturity Date [Line Items]  
Carrying Value 25,964
Weighted- Average Yield 2.39%
After One Year to Five Years - Carrying Value 25,964
After One Year to Five Years - Weighted- Average Yield 2.39%
U.S. agency debentures
  
Investments Classified by Contractual Maturity Date [Line Items]  
Carrying Value 2,874,932
Weighted- Average Yield 1.60%
One Year or Less - Carrying Value 38,183
One Year or Less - Weighted- Average Yield 2.19%
After One Year to Five Years - Carrying Value 2,659,334
After One Year to Five Years - Weighted- Average Yield 1.55%
After Five Years to Ten Years - Carrying Value 177,415
After Five Years to Ten Years - Weighted- Average Yield 2.16%
Residential mortgage-backed securities | Agency-issued commercial mortgage-backed securities
  
Investments Classified by Contractual Maturity Date [Line Items]  
Carrying Value 1,564,286
Weighted- Average Yield 2.59%
After Five Years to Ten Years - Carrying Value 1,434,122
After Five Years to Ten Years - Weighted- Average Yield 2.50%
After Ten Years - Carrying Value 130,164
After Ten Years - Weighted- Average Yield 3.53%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
  
Investments Classified by Contractual Maturity Date [Line Items]  
Carrying Value 3,373,760
Weighted- Average Yield 2.41%
After Ten Years - Carrying Value 3,373,760
After Ten Years - Weighted- Average Yield 2.41%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
  
Investments Classified by Contractual Maturity Date [Line Items]  
Carrying Value 2,413,378
Weighted- Average Yield 0.70%
After Ten Years - Carrying Value 2,413,378
After Ten Years - Weighted- Average Yield 0.70%
Municipal bonds and notes
  
Investments Classified by Contractual Maturity Date [Line Items]  
Carrying Value 100,498
Weighted- Average Yield 6.00%
One Year or Less - Carrying Value 559
One Year or Less - Weighted- Average Yield 5.38%
After One Year to Five Years - Carrying Value 12,089
After One Year to Five Years - Weighted- Average Yield 5.48%
After Five Years to Ten Years - Carrying Value 50,242
After Five Years to Ten Years - Weighted- Average Yield 5.97%
After Ten Years - Carrying Value $ 37,608
After Ten Years - Weighted- Average Yield 6.22%