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Debt Securities Available-for-Sale - Summary of Expected Maturity Distribution of Debt Securities Available-for-Sale, Other than Mortgage-Backed Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Amortized cost
Amortized cost	$ 898,434	$ 820,801
Estimated fair value
Estimated fair value	894,272	$ 808,031
Other Than Mortgage Backed Securities
Amortized cost
Due in one year or less	44,901
Due after one year through five years	176,033
Due after five years through ten years	4,935
Amortized cost	225,869
Estimated fair value
Due in one year or less	44,947
Due after one year through five years	176,354
Due after one year through five years	4,885
Estimated fair value	$ 226,186

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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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