Fair Value Measurements - Book Value and Estimated Fair Value of Bank's Significant Financial Instruments Not Recorded at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financial Assets:
Debt securities held-to-maturity	$ 1,099,745	$ 968,466
Restricted equity investments	52,199	51,705
Financial Liabilities:
Time deposits	1,110,758	1,372,783
Securities sold under agreements to repurchase with retail customers	134,465	128,454
Book Value
Financial Assets:
Cash and due from banks	1,173,665	1,272,134
Debt securities held-to-maturity	1,082,326	937,253
Restricted equity investments	52,199	51,705
Loans receivable, net and loans held-for-sale	7,863,765	7,750,381
Financial Liabilities:
Deposits other than time deposits	8,392,054	8,054,833
Time deposits	1,110,758	1,372,783
Other borrowings	228,176	235,471
Securities sold under agreements to repurchase with retail customers	134,465	128,454
Level 1 Inputs
Financial Assets:
Cash and due from banks	1,173,665	1,272,134
Debt securities held-to-maturity	0	0
Restricted equity investments	0	0
Loans receivable, net and loans held-for-sale	0	0
Financial Liabilities:
Deposits other than time deposits	0	0
Time deposits	0	0
Other borrowings	0	0
Securities sold under agreements to repurchase with retail customers	134,465	128,454
Level 2 Inputs
Financial Assets:
Cash and due from banks	0	0
Debt securities held-to-maturity	1,081,590	952,365
Restricted equity investments	0	0
Loans receivable, net and loans held-for-sale	0	0
Financial Liabilities:
Deposits other than time deposits	8,392,054	8,054,833
Time deposits	1,118,867	1,383,173
Other borrowings	241,122	251,798
Securities sold under agreements to repurchase with retail customers	0	0
Level 3 Inputs
Financial Assets:
Cash and due from banks	0	0
Debt securities held-to-maturity	18,155	16,101
Restricted equity investments	52,199	51,705
Loans receivable, net and loans held-for-sale	7,822,353	7,806,743
Financial Liabilities:
Deposits other than time deposits	0	0
Time deposits	0	0
Other borrowings	0	0
Securities sold under agreements to repurchase with retail customers	$ 0	$ 0