Investment Securities - Summary of Remaining Contractual Principal Maturities for Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	$ 14,131,154	$ 14,115,272
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	468,152	524,700
Residential mortgage-backed securities \| Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	6,973,684	7,132,152
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations—fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	1,512,524	1,600,122
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations—variable rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	169,122	178,446
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,743,262	2,812,021
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,264,410	$ 1,867,831
After One Year to Five Years, Amortized Cost	134,007
After Five Years to Ten Years, Amortized Cost	441,995
Held-to-maturity Securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	14,131,154
Amortized Cost	13,574,519
One Year or Less, Amortized Cost	45,194
One Year or Less, Fair Value	45,413
After One Year to Five Years, Amortized Cost	327,774
After One Year to Five Years, Fair Value	334,520
After Five Years to Ten Years, Amortized Cost	2,198,892
After Five Years to Ten Years, Fair Value	2,277,467
After Ten Years, Amortized Cost	11,002,659
After Ten Years, Fair Value	$ 11,473,754
Held-to-maturity Securities \| Lower Limit
Schedule of Held-to-maturity Securities [Line Items]
Mortgage-backed securities contractual maturities (in years)	10 years
Held-to-maturity Securities \| Upper Limit
Schedule of Held-to-maturity Securities [Line Items]
Mortgage-backed securities contractual maturities (in years)	30 years
Held-to-maturity Securities \| U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	$ 468,152
Amortized Cost	453,353
One Year or Less, Amortized Cost	2,622
One Year or Less, Fair Value	2,673
After One Year to Five Years, Amortized Cost	142,990
After One Year to Five Years, Fair Value	146,270
After Five Years to Ten Years, Amortized Cost	307,741
After Five Years to Ten Years, Fair Value	319,209
After Ten Years, Amortized Cost	0
After Ten Years, Fair Value	0
Held-to-maturity Securities \| Residential mortgage-backed securities \| Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	6,973,684
Amortized Cost	6,662,878
One Year or Less, Amortized Cost	11,853
One Year or Less, Fair Value	11,983
After One Year to Five Years, Amortized Cost	50,777
After One Year to Five Years, Fair Value	51,678
After Five Years to Ten Years, Amortized Cost	685,374
After Five Years to Ten Years, Fair Value	702,253
After Ten Years, Amortized Cost	5,914,874
After Ten Years, Fair Value	6,207,770
Held-to-maturity Securities \| Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations—fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	1,512,524
Amortized Cost	1,482,205
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	0
After One Year to Five Years, Fair Value	0
After Five Years to Ten Years, Amortized Cost	661,217
After Five Years to Ten Years, Fair Value	674,788
After Ten Years, Amortized Cost	820,988
After Ten Years, Fair Value	837,736
Held-to-maturity Securities \| Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations—variable rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	169,122
Amortized Cost	170,774
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	0
After One Year to Five Years, Fair Value	0
After Five Years to Ten Years, Amortized Cost	0
After Five Years to Ten Years, Fair Value	0
After Ten Years, Amortized Cost	170,774
After Ten Years, Fair Value	169,122
Held-to-maturity Securities \| Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,743,262
Amortized Cost	2,644,968
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	0
After One Year to Five Years, Fair Value	0
After Five Years to Ten Years, Amortized Cost	102,565
After Five Years to Ten Years, Fair Value	118,381
After Ten Years, Amortized Cost	2,542,403
After Ten Years, Fair Value	2,624,881
Held-to-maturity Securities \| Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,264,410
Amortized Cost	2,160,341
One Year or Less, Amortized Cost	30,719
One Year or Less, Fair Value	30,757
After One Year to Five Years, Fair Value	136,572
After Five Years to Ten Years, Fair Value	462,836
After Ten Years, Amortized Cost	1,553,620
After Ten Years, Fair Value	$ 1,634,245

The period (in years) that is typical for contractual maturities of mortgage-backed securities.

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Amount, before other-than-temporary impairment (OTTI) accretion, of investment in debt securities measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Amount of investment in debt security measured at amortized cost (held-to-maturity), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Fair value of investment in debt security measured at amortized cost (held-to-maturity), with single maturity date and allocated without single maturity date, maturing in next fiscal year through fifth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Amount of investment in debt security measured at amortized cost (held-to-maturity), with single maturity date and allocated without single maturity date, maturing in next fiscal year through fifth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Fair value of investment in debt security measured at amortized cost (held-to-maturity), with single maturity date and allocated without single maturity date, maturing after tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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