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, 2017 USD ($)
Investments Classified By Contractual Maturity Date [Line Items]
Carrying Value	$ 11,048,395
One Year or Less - Carrying Value	2,448,760
After One Year to Five Years - Carrying Value	5,958,870
After Five Years to Ten Years - Carrying Value	88,425
After Ten Years - Carrying Value	2,552,340
U.S. treasury securities
Investments Classified By Contractual Maturity Date [Line Items]
Carrying Value	6,840,502
One Year or Less - Carrying Value	1,967,480
After One Year to Five Years - Carrying Value	4,873,022
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	1,567,128
One Year or Less - Carrying Value	481,280
After One Year to Five Years - Carrying Value	1,085,848
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	2,267,035
One Year or Less - Carrying Value	0
After One Year to Five Years - Carrying Value	0
After Five Years to Ten Years - Carrying Value	88,425
After Ten Years - Carrying Value	2,178,610
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investments Classified By Contractual Maturity Date [Line Items]
Carrying Value	373,730
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	$ 373,730