Investment Securities - Amortized Cost and Fair Value of Available for Sale Securities and Held to Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Available for Sale, Amortized Cost
Due in one year or less	$ 1,134
Due after one through five years	5,031
Due after five through ten years	74,745
Due after ten years	817,161
Amortized Cost	898,071
Amortized Cost	1,740,420	$ 1,149,727
Available for Sale, Fair Value
Due in one year or less	1,136
Due after one through five years	5,141
Due after five through ten years	76,920
Due after ten years	855,505
Fair Value	938,702
Total Investment Securities	1,790,025	1,142,195
Held to Maturity, Amortized Cost
Due in one year or less	9,920
Due after one through five years	45,197
Due after five through ten years	84,153
Due after ten years	231,964
Amortized Cost	371,234
Amortized Cost	806,038	490,387
Held to Maturity, Fair Value
Due in one year or less	10,105
Due after one through five years	45,654
Due after five through ten years	88,844
Due after ten years	241,639
Fair Value	386,242
Total Investment Securities	827,566	$ 489,217
U.S. Government-sponsored mortgage-backed securities
Available for Sale, Amortized Cost
U.S. Government-sponsored mortgage-backed securities	842,349
Available for Sale, Fair Value
Without single maturity date	851,323
Held to Maturity, Amortized Cost
Without single maturity date	434,804
Held to Maturity, Fair Value
Without single maturity date	$ 441,324

Available-For-Sale Securities, Debt Maturity, with Single Maturity Date, Amortized Cost

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Available-For-Sale Securities, Debt Maturity, with Single Maturity Date, Fair Value

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Carrying amount of debt securities accounted for as held-to-maturity, net of adjustments including, but not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized and fair value hedge accounting adjustments, maturing in the next rolling twelve months following the latest balance sheet presented.

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Carrying amount of debt securities accounted for as held-to-maturity, net of adjustments including, but not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized and fair value hedge accounting adjustments, maturing in the sixth rolling twelve months through the tenth rolling twelve months following the latest balance sheet presented.

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Amortized cost 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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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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Amortized cost 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 next rolling fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Amortized cost 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 after rolling tenth fiscal year following latest fiscal year. For 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), without single maturity date and not allocated over maturity grouping.

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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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Amount of investment in debt security 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 next rolling fiscal year following latest fiscal year. For 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 rolling fiscal year following latest fiscal year. For 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 sixth through tenth rolling fiscal year following latest fiscal year. For 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 second through fifth rolling fiscal year following latest fiscal year. For 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), without single maturity date and not allocated over maturity grouping.

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

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