Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Activities
Net income	$ 49,599	$ 49,336	$ 44,384	$ 38,559	$ 14,956	$ 36,479	$ 33,687	$ 31,255	$ 181,878	$ 116,377	$ 121,131
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of purchase accounting adjustments									(3,963)	(5,131)	(2,252)
Gain on sale of investments									1,113	660	1,463
Stock-based compensation, net of tax benefits									3,122	2,952	1,844
Net cash provided by operating activities									280,711	254,741	193,087
Investing Activities
Sales of available-for-sale debt securities									226,842	247,748	62,817
Net (increase) decrease of premises and equipment									(18,637)	(10,128)	(8,306)
Net cash used in investing activities									(872,420)	23,655	(421,313)
Financing Activities
Net increase in other borrowed funds									(5,059)	3,784	(521)
Cash dividends paid									(85,493)	(111,720)	(84,040)
Tax withholding payments for stock-based compensation									(1,190)	(1,531)	(600)
Net cash used in financing activities									595,495	(230,933)	187,514
Net increase (decrease) in cash, cash equivalents and restricted cash									3,786	47,463	(40,712)
Cash, cash equivalents and restricted cash at beginning of period				200,004				152,541	200,004	152,541	193,253
Cash, cash equivalents and restricted cash at end of period	203,790				200,004				203,790	200,004	152,541
Parent Company
Operating Activities
Net income									181,878	116,377	121,131
Adjustments to reconcile net income to net cash provided by operating activities:
Subsidiary income in excess of dividends distributed									(39,798)	(8,156)	(19,548)
Amortization of purchase accounting adjustments									13	143	143
Gain on sale of investments									0	(3)	0
Stock-based compensation, net of tax benefits									1,219	1,460	804
Net change in other assets and other liabilities									(3,222)	5,051	(297)
Net cash provided by operating activities									140,090	114,872	102,233
Investing Activities
Sales of available-for-sale debt securities									0	27	0
Net (increase) decrease of premises and equipment									(300)	(79)	771
Proceeds from sale of non-marketable equity securities									0	114	55
Equity contribution to subsidiaries									(24,989)	(17,565)	(3,475)
Net cash used in investing activities									(25,289)	(17,503)	(2,649)
Financing Activities
Net increase in other borrowed funds									(11,543)	0	0
Cash dividends paid									(85,493)	(111,720)	(84,040)
Tax withholding payments for stock-based compensation									(1,190)	(1,531)	(600)
Net cash used in financing activities									(98,226)	(113,251)	(84,640)
Net increase (decrease) in cash, cash equivalents and restricted cash									16,575	(15,882)	14,944
Cash, cash equivalents and restricted cash at beginning of period				$ 47,724				$ 63,606	47,724	63,606	48,662
Cash, cash equivalents and restricted cash at end of period	$ 64,299				$ 47,724				$ 64,299	$ 47,724	$ 63,606