Investment Securities - Schedule of Contractual Maturities of Securities Available-for-Sale (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Investments Debt And Equity Securities [Abstract]
Available-for-Sale, Due in one year or less, Amortized Cost	$ 39,560
Available-for-Sale, Due after one year through five years, Amortized Cost	128,883
Available-for-Sale, Due after five years through ten years, Amortized Cost	50,865
Available-for-Sale, Due after ten years, Amortized Cost	229,285
Available-for-Sale, Mortgage-backed securities, Amortized Cost	1,311,118
Available-for-Sale, Total, Amortized Cost	1,759,711
Available-for-Sale, Due in one year or less, Estimated Fair Value	39,596
Available-for-Sale, Due after one year through five years, Estimated Fair Value	126,188
Available-for-Sale, Due after five years through ten years, Estimated Fair Value	51,521
Available-for-Sale, Due after ten years, Estimated Fair Value	228,232
Available-for-Sale, Mortgage-backed securities, Estimated Fair Value	1,309,302
Available-for-Sale, Total, Estimated Fair Value	$ 1,754,839