Investment Securities - Summary of Remaining Contractual Principal Maturities and Fully Taxable Equivalent Yields on Debt Securities Classified as Available for Sale (Detail) - Available-for-sale Securities $ in Thousands	Dec. 31, 2016 USD ($)
Investments Classified By Contractual Maturity Date [Line Items]
Carrying Value	$ 12,614,814
One Year or Less - Carrying Value	2,725,058
After One Year to Five Years - Carrying Value	8,262,808
After Five Years to Ten Years - Carrying Value	680,688
After Ten Years - Carrying Value	946,260
U.S. treasury securities
Investments Classified By Contractual Maturity Date [Line Items]
Carrying Value	8,909,491
One Year or Less - Carrying Value	2,224,538
After One Year to Five Years - Carrying Value	6,684,953
After Five Years to Ten Years - Carrying Value	0
After Ten Years - Carrying Value	0
U.S. agency debentures
Investments Classified By Contractual Maturity Date [Line Items]
Carrying Value	2,078,375
One Year or Less - Carrying Value	500,520
After One Year to Five Years - Carrying Value	1,577,855
After Five Years to Ten Years - Carrying Value	0
After Ten Years - Carrying Value	0
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Investments Classified By Contractual Maturity Date [Line Items]
Carrying Value	1,152,665
One Year or Less - Carrying Value	0
After One Year to Five Years - Carrying Value	0
After Five Years to Ten Years - Carrying Value	680,688
After Ten Years - Carrying Value	471,977
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investments Classified By Contractual Maturity Date [Line Items]
Carrying Value	474,283
One Year or Less - Carrying Value	0
After One Year to Five Years - Carrying Value	0
After Five Years to Ten Years - Carrying Value	0
After Ten Years - Carrying Value	$ 474,283