Investment Securities - Contractual Maturities of Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Due in one year	$ 21,825
Due after one year through five years	231,676
Due after five years through ten years	295,711
Due after ten years	263,379
Residential mortgage-backed securities	1,259,772
Amortized Cost	2,072,363	$ 1,842,691
Fair Value
Due in one year	21,850
Due after one year through five years	233,954
Due after five years through ten years	300,013
Due after ten years	248,781
Residential mortgage-backed securities	1,211,756
Total investment securities held to maturity	$ 2,016,354	$ 1,837,620

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

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