Investment Securities - Summary of Remaining Contractual Principal Maturities for Available-for-Sale Securities (Details) $ in Thousands	3 Months Ended
Mar. 31, 2019 USD ($)
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	$ 6,755,094
One Year or Less	1,809,189
After One Year to Five Years	2,620,959
After Five Years to Ten Years	415,314
After Ten Years	$ 1,909,632
Available-for-sale Securities \| Lower Limit
Debt Securities, Available-for-sale [Line Items]
Mortgage-backed securities contractual maturities (in years)	10 years
Available-for-sale Securities \| Upper Limit
Debt Securities, Available-for-sale [Line Items]
Mortgage-backed securities contractual maturities (in years)	30 years
Available-for-sale Securities \| U.S. treasury securities
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	$ 3,767,410
One Year or Less	1,165,910
After One Year to Five Years	2,194,840
After Five Years to Ten Years	406,660
After Ten Years	0
Available-for-sale Securities \| U.S. agency debentures
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	1,063,713
One Year or Less	643,279
After One Year to Five Years	420,434
After Five Years to Ten Years	0
After Ten Years	0
Available-for-sale Securities \| Foreign government debt securities
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	5,685
One Year or Less	0
After One Year to Five Years	5,685
After Five Years to Ten Years	0
After Ten Years	0
Available-for-sale Securities \| Residential mortgage-backed securities \| Fixed rate
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	1,839,883
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	8,654
After Ten Years	1,831,229
Available-for-sale Securities \| Residential mortgage-backed securities \| Variable rate
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total	78,403
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	0
After Ten Years	$ 78,403

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

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), 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 fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second 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 fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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