Noninterest Income (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
Noninterest Income
Retirement and benefits									$ 63,811	$ 63,316	$ 62,390
Wealth management									15,502	14,900	13,953
Mortgage banking									25,805	17,630	19,748
Service charges on deposit accounts									1,772	1,808	1,854
Net gains (losses) on investment securities									357	85	(13)
Interchange fees									1,972	2,005	1,997
Bank-owned life insurance income									803	804	820
Misc. transactional fees									1,236	1,106	1,070
Other Noninterest Income									2,936	1,096	1,226
Total noninterest income	$ 29,556	$ 29,580	$ 29,984	$ 25,074	$ 26,141	$ 26,258	$ 26,094	$ 24,256	$ 114,194	$ 102,749	$ 103,045