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, 2020	Dec. 31, 2019
Amortized cost
Amortized cost	$ 1,046,738	$ 1,131,970
Estimated fair value
Estimated fair value	1,061,443	$ 1,138,352
Other Than Mortgage Backed Securities
Amortized cost
Due in one year or less	77,203
Due after one year through five years	50,718
Amortized cost	127,921
Estimated fair value
Due in one year or less	76,941
Due after one year through five years	50,004
Estimated fair value	$ 126,945