Condensed Financial Statements of Parent Company (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
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	$ 43,009	$ 42,592	$ 42,681	$ 42,758	$ 44,449	$ 46,378	$ 47,530	$ 51,232	$ 171,040	$ 189,589	$ 139,445
Adjustments to reconcile net income to net cash from operating activities:
Stock-based compensation expense									5,282	3,659	3,161
Net change in fair value of equity investments with readily determinable fair value									(1,288)	(1,449)	0
Change in other assets									(23,157)	(6,138)	(1,987)
Net cash provided by operating activities									183,855	219,947	203,539
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from sale of equity investments									2,570	0	0
Net cash used in investing activities									(36,823)	(1,147,465)	(766,953)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from convertible notes, net of issuance fees									0	212,920	0
Purchase of treasury stock									13,223	150,000	0
Net cash provided by financing activities									91,929	895,124	618,080
NET CHANGE IN CASH AND CASH EQUIVALENTS									238,961	(32,394)	54,666
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR				459,606				492,000	459,606	492,000	437,334
CASH AND CASH EQUIVALENTS, END OF YEAR	698,567				459,606				698,567	459,606	492,000
Parent Company
CASH FLOWS FROM OPERATING ACTIVITIES
Net income									171,040	189,589	139,445
Adjustments to reconcile net income to net cash from operating activities:
Amortization									6,021	4,118	1,045
Stock-based compensation expense									0	300	523
Net change in fair value of equity investments with readily determinable fair value									(175)	(525)	0
Change in other assets									(999)	4,534	665
Change in accounts payable and other liabilities									257	653	(17)
Equity in undistributed earnings of bank subsidiary									(65,713)	(191,161)	(76,397)
Net cash provided by operating activities									110,431	7,508	65,264
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from sale of equity investments									700	0	0
Net cash used in investing activities									700	0	0
CASH FLOWS FROM FINANCING ACTIVITIES:
Issuance of additional stock pursuant to various stock plans									12	469	1,865
Proceeds from convertible notes, net of issuance fees									0	212,920	0
Purchase of treasury stock									(13,820)	(150,000)	0
Payments of cash dividends									(70,935)	(71,631)	(67,661)
Net cash provided by financing activities									(84,743)	(8,242)	(65,796)
NET CHANGE IN CASH AND CASH EQUIVALENTS									26,388	(734)	(532)
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR				$ 12,593				$ 13,327	12,593	13,327	13,859
CASH AND CASH EQUIVALENTS, END OF YEAR	$ 38,981				$ 12,593				$ 38,981	$ 12,593	$ 13,327