Investment Securities - Summary of Unrealized Losses on Available-for-Sale Securities (Detail) $ in Thousands	Jun. 30, 2018 USD ($) Investment	Dec. 31, 2017 USD ($) Investment
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months - Fair Value of Investments	$ 7,396,280	$ 8,913,199
Less than 12 months - Unrealized Losses	(112,453)	(51,432)
12 months or longer - Fair Value of Investments	1,516,249	713,348
12 months or longer - Unrealized Losses	(16,938)	(5,752)
Fair Value of Investments	8,912,529	9,626,547
Unrealized Losses	$ (129,391)	$ (57,184)
Number of investments in unrealized loss position \| Investment	264	268
Number of investments with unrealized losses greater than 12 months \| Investment	64	46
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months - Fair Value of Investments	$ 4,492,009	$ 5,968,914
Less than 12 months - Unrealized Losses	(48,430)	(23,397)
12 months or longer - Fair Value of Investments	845,938	323,966
12 months or longer - Unrealized Losses	(5,565)	(2,282)
Fair Value of Investments	5,337,947	6,292,880
Unrealized Losses	(53,995)	(25,679)
U.S. agency debentures
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months - Fair Value of Investments	1,027,612	736,541
Less than 12 months - Unrealized Losses	(3,984)	(2,289)
12 months or longer - Fair Value of Investments	458,252	336,196
12 months or longer - Unrealized Losses	(5,809)	(3,347)
Fair Value of Investments	1,485,864	1,072,737
Unrealized Losses	(9,793)	(5,636)
Residential mortgage-backed securities \| Fixed rate \| Agency-issued collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months - Fair Value of Investments	1,857,591	2,193,277
Less than 12 months - Unrealized Losses	(60,030)	(25,534)
12 months or longer - Fair Value of Investments	166,002	0
12 months or longer - Unrealized Losses	(5,447)	0
Fair Value of Investments	2,023,593	2,193,277
Unrealized Losses	(65,477)	(25,534)
Residential mortgage-backed securities \| Variable rate \| Agency-issued collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months - Fair Value of Investments	19,068	13,843
Less than 12 months - Unrealized Losses	(9)	(3)
12 months or longer - Fair Value of Investments	46,057	53,186
12 months or longer - Unrealized Losses	(117)	(123)
Fair Value of Investments	65,125	67,029
Unrealized Losses	$ (126)	(126)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months - Fair Value of Investments		624
Less than 12 months - Unrealized Losses		(209)
12 months or longer - Fair Value of Investments		0
12 months or longer - Unrealized Losses		0
Fair Value of Investments		624
Unrealized Losses		$ (209)

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

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