Parent Company Financial Information- Cash flow (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income	$ 132,217	$ 142,943	$ 152,276
Adjustments to reconcile net income to net cash provided by operating activities:
Net tax benefits from stock compensation	118	(48)	110
Securities premium amortization (discount accretion), net	8,196	5,186	4,445
Increase in other assets	(28,626)	(21,421)	(16,643)
(Decrease) increase in other liabilities	9,826	19,809	2,061
Net cash provided by operating activities	133,139	132,684	165,455
Cash Flows From Investing Activities:
Purchases of available-for-sale investment securities	(739,955)	(374,648)	(331,884)
Proceeds from maturities of available-for-sale securities	302,471	214,204	93,848
Net cash used in investing activities	(557,676)	(637,127)	(784,801)
Cash Flows From Financing Activities:
Proceeds from exercise of stock options	63	332	776
Proceeds from employee stock purchase plan	760	782	808
Common stock repurchased	61,432	54,903	0
Cash dividends paid	28,330	22,332	0
Net cash provided by financing activities	1,971,619	424,552	750,737
Net (Decrease) Increase In Cash and Cash Equivalents	1,547,082	(79,891)	131,391
Cash and Cash Equivalents at Beginning of Period	241,973	321,864	190,473
Cash and Cash Equivalents at End of Period	1,789,055	241,973	321,864
Depreciation and amortization	4,696	6,174	6,969
Parent Company
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income	132,217	142,943	152,276
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed income of subsidiary	(6,191)	(70,396)	(163,393)
Net tax benefits from stock compensation	118	10	110
Securities premium amortization (discount accretion), net	6	2	0
Increase in other assets	(48,966)	(21,447)	(2,508)
(Decrease) increase in other liabilities	6,823	2,460	(61)
Net cash provided by operating activities	84,397	53,572	(13,576)
Cash Flows From Investing Activities:
Purchases of available-for-sale investment securities	(10,000)	(7,030)	0
Proceeds from maturities of available-for-sale securities	613	0	0
Investment in subsidiary (net)	0	0	6,892
Net cash used in investing activities	(9,387)	(7,030)	6,892
Cash Flows From Financing Activities:
Proceeds from exercise of stock options	63	332	776
Proceeds from employee stock purchase plan	760	782	808
Common stock repurchased	(61,432)	(54,903)	0
Cash dividends paid	(28,330)	(22,332)	0
Net cash provided by financing activities	(88,939)	(76,121)	1,584
Net (Decrease) Increase In Cash and Cash Equivalents	(13,929)	(29,579)	(5,100)
Cash and Cash Equivalents at Beginning of Period	43,204	72,783	77,883
Cash and Cash Equivalents at End of Period	29,275	43,204	72,783
Depreciation and amortization	$ 390	$ 0	$ 0