Condensed Financial Information (Parent Company Only) - Condensed Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Income (Expense)
Other income									$ 1,513	$ 2,394	$ 2,141
Interest expense	$ (9,612)	$ (6,749)	$ (7,348)	$ (10,729)	$ (11,879)	$ (12,248)	$ (12,321)	$ (11,093)	(34,438)	(47,541)	(31,599)
Employee benefit expense									(71,082)	(65,206)	(56,623)
Income Before Tax									78,369	79,841	63,560
Income Tax Benefit									(9,870)	(13,303)	(10,443)
Net Income Available to Common Shareholders									68,499	66,538	53,117
Parent Company
Operating Income (Expense)
Dividend income from subsidiaries									61,400	46,150	46,950
Other income									106	0	0
Interest expense									(4,483)	(3,209)	(2,475)
Employee benefit expense									(1,997)	(1,687)	(1,423)
Other expense									(517)	(416)	(357)
Income Before Undistributed Income of Subsidiaries									54,509	40,838	42,695
Undistributed Income of Subsidiaries									13,131	25,053	9,643
Income Before Tax									67,640	65,891	52,338
Income Tax Benefit									859	647	779
Net Income Available to Common Shareholders									$ 68,499	$ 66,538	$ 53,117