Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Activities
Net income	$ 57,410	$ 51,610	$ 52,392	$ 49,132	$ 49,599	$ 49,336	$ 44,384	$ 38,559	$ 210,544	$ 181,878	$ 116,377
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation, net of tax benefits									7,475	3,122	2,952
Net cash provided by operating activities									226,649	280,711	254,741
Investing Activities
Sales of available-for-sale debt securities									712,113	226,842	247,748
Net additions of premises and equipment									(16,398)	(18,637)	(10,128)
Net cash used in investing activities									(42,682)	(872,420)	23,655
Financing Activities
Net increase in other borrowed funds									14,109	(5,059)	3,784
Cash dividends paid									(124,468)	(85,493)	(111,720)
Tax withholding payments for stock-based compensation									(1,293)	(1,214)	(1,531)
Proceeds from stock option exercises									2,896	24	0
Net cash used in financing activities									(56,796)	595,495	(230,933)
Net (decrease) increase in cash, cash equivalents and restricted cash									127,171	3,786	47,463
Cash, cash equivalents and restricted cash at beginning of period				203,790				200,004	203,790	200,004	152,541
Cash, cash equivalents and restricted cash at end of period	330,961				203,790				330,961	203,790	200,004
Parent Company
Operating Activities
Net income									210,544	181,878	116,377
Adjustments to reconcile net income to net cash provided by operating activities:
Subsidiary income in excess of dividends distributed									(83,806)	(39,798)	(8,156)
Stock-based compensation, net of tax benefits									1,320	1,219	1,460
Net change in other assets and other liabilities									1,854	(3,209)	5,191
Net cash provided by operating activities									129,912	140,090	114,872
Investing Activities
Sales of available-for-sale debt securities									0	0	27
Net additions of premises and equipment									(114)	(300)	(79)
Proceeds from sale of marketable equity securities									89	0	114
Equity contribution to subsidiaries									(13,485)	(24,989)	(17,565)
Net cash used in investing activities									(13,510)	(25,289)	(17,503)
Financing Activities
Net increase in other borrowed funds									0	(11,543)	0
Cash dividends paid									(124,468)	(85,493)	(111,720)
Tax withholding payments for stock-based compensation									(1,293)	(1,214)	(1,531)
Proceeds from stock option exercises									2,896	24	0
Net cash used in financing activities									(122,865)	(98,226)	(113,251)
Net (decrease) increase in cash, cash equivalents and restricted cash									(6,463)	16,575	(15,882)
Cash, cash equivalents and restricted cash at beginning of period				$ 64,299				$ 47,724	64,299	47,724	63,606
Cash, cash equivalents and restricted cash at end of period	$ 57,836				$ 64,299				$ 57,836	$ 64,299	$ 47,724