Summary of the Remaining Contractual Principal Maturities and Fully Taxable Equivalent Yields on Debt Securities Classified as Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2011
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	$ 9,580,242
Weighted- Average Yield	1.81%
One Year or Less - Carrying Value	109,152
One Year or Less - Weighted- Average Yield	2.02%
After One Year to Five Years - Carrying Value	3,096,352
After One Year to Five Years - Weighted- Average Yield	1.50%
After Five Years to Ten Years - Carrying Value	1,173,360
After Five Years to Ten Years - Weighted- Average Yield	2.86%
After Ten Years - Carrying Value	5,201,378
After Ten Years - Weighted- Average Yield	1.74%
U.S. treasury securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	26,228
Weighted- Average Yield	2.39%
After One Year to Five Years - Carrying Value	26,228
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	3,194,364
Weighted- Average Yield	1.52%
One Year or Less - Carrying Value	108,583
One Year or Less - Weighted- Average Yield	2.00%
After One Year to Five Years - Carrying Value	3,060,089
After One Year to Five Years - Weighted- Average Yield	1.48%
After Five Years to Ten Years - Carrying Value	25,692
After Five Years to Ten Years - Weighted- Average Yield	4.07%
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	1,342,050
Weighted- Average Yield	2.76%
After Five Years to Ten Years - Carrying Value	1,102,807
After Five Years to Ten Years - Weighted- Average Yield	2.71%
After Ten Years - Carrying Value	239,243
After Ten Years - Weighted- Average Yield	2.96%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	2,192,636
Weighted- Average Yield	2.80%
After Ten Years - Carrying Value	2,192,636
After Ten Years - Weighted- Average Yield	2.80%
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	2,702,630
Weighted- Average Yield	0.70%
After Ten Years - Carrying Value	2,702,630
After Ten Years - Weighted- Average Yield	0.70%
Commercial mortgage-backed securities
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	25,736
Weighted- Average Yield	2.70%
After Ten Years - Carrying Value	25,736
After Ten Years - Weighted- Average Yield	2.70%
Municipal bonds and notes
Investments Classified by Contractual Maturity Date [Line Items]
Carrying Value	96,598
Weighted- Average Yield	6.00%
One Year or Less - Carrying Value	569
One Year or Less - Weighted- Average Yield	5.38%
After One Year to Five Years - Carrying Value	10,035
After One Year to Five Years - Weighted- Average Yield	5.45%
After Five Years to Ten Years - Carrying Value	44,861
After Five Years to Ten Years - Weighted- Average Yield	5.95%
After Ten Years - Carrying Value	$ 41,133
After Ten Years - Weighted- Average Yield	6.20%