Investment Securities - Major Components of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Investment Holdings [Line Items]
Available-for-sale Securities, Gross Realized Gains (Losses), Sale Proceeds	$ 0	$ 2,078
Amortized Cost	10,189,071		$ 11,131,008
Unrealized Gains	1,793		46,840
Unrealized Losses	(110,480)		(57,184)
Carrying Value	10,080,384		11,120,664
Net gains (losses) on non-marketable and other equity securities:	0	675
Available-for-sale Securities, Gross Realized Losses	0	(67)
Gain (Loss) on Sale of Securities, Net	0	$ 608
U.S. treasury securities
Investment Holdings [Line Items]
Amortized Cost	6,066,397		6,865,068
Unrealized Gains	0		1,113
Unrealized Losses	(49,520)		(25,679)
Carrying Value	6,016,877		6,840,502
U.S. agency debentures
Investment Holdings [Line Items]
Amortized Cost	1,569,943		1,569,195
Unrealized Gains	321		3,569
Unrealized Losses	(8,430)		(5,636)
Carrying Value	1,561,834		1,567,128
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Fixed rate
Investment Holdings [Line Items]
Carrying Value	2,146,173		2,267,035
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Variable rate
Investment Holdings [Line Items]
Carrying Value	355,500		373,730
Equity securities
Investment Holdings [Line Items]
Amortized Cost			31,953
Unrealized Gains			40,525
Unrealized Losses			(209)
Carrying Value			72,269
Variable rate \| Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations
Investment Holdings [Line Items]
Amortized Cost	354,252		372,481
Unrealized Gains	1,375		1,375
Unrealized Losses	(127)		(126)
Fixed rate \| Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations
Investment Holdings [Line Items]
Amortized Cost	2,198,479		2,292,311
Unrealized Gains	97		258
Unrealized Losses	$ (52,403)		$ (25,534)

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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The total amount of proceeds received for the sale of securities, categorized neither as held-to-maturity nor trading securities, during the reporting period.

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