Loans - Nonaccrual loans (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Loans
Total loans on nonaccrual status	$ 121,810,000		$ 121,810,000	$ 33,141,000
Greater than 90 Days Accruing	1,471,000		1,471,000
Non-accrual with no allowance	61,601,000		61,601,000
Addition of loans formerly accounted for as credit impaired loans			21,000,000.0
Loans	25,064,539,000		$ 25,064,539,000	11,313,113,000	$ 11,167,979,000
Number of months generally required to return to accruing status			6 months
Increase in overall collateral dependent loans	2,300,000
Amount of decrease in collateral dependent loans		$ 11,100,000
Loans individually evaluated for impairment	1,000,000.0	$ 500,000	$ 1,000,000.0
CSFL
Loans
Increase in overall collateral dependent loans	17,600,000
Construction and land development
Loans
Total loans on nonaccrual status	2,967,000		2,967,000	1,193,000
Non-accrual with no allowance	1,803,000		1,803,000
Consumer Owner Occupied Loans
Loans
Total loans on nonaccrual status	37,881,000		37,881,000	9,718,000
Greater than 90 Days Accruing	310,000		310,000
Non-accrual with no allowance	20,224,000		20,224,000
Other income producing property
Loans
Total loans on nonaccrual status	7,945,000		7,945,000	1,947,000
Greater than 90 Days Accruing	38,000		38,000
Non-accrual with no allowance	5,305,000		5,305,000
Acquired credit impaired loans | CSFL
Loans
Loans	69,300,000		69,300,000
Commercial loans | Commercial non-owner occupied
Loans
Total loans on nonaccrual status	9,643,000		9,643,000	1,154,000
Non-accrual with no allowance	5,351,000		5,351,000
Commercial loans | Commercial non-owner occupied real estate
Loans
Total loans on nonaccrual status	12,610,000		12,610,000	2,347,000
Non-accrual with no allowance	7,154,000		7,154,000
Commercial loans | Commercial owner occupied real estate loan
Loans
Total loans on nonaccrual status	36,590,000		36,590,000	4,385,000
Greater than 90 Days Accruing	16,000		16,000
Non-accrual with no allowance	17,096,000		17,096,000
Commercial loans | Commercial and industrial
Loans
Total loans on nonaccrual status	7,210,000		7,210,000	6,913,000
Greater than 90 Days Accruing	1,107,000		1,107,000
Non-accrual with no allowance	5,154,000		5,154,000
Consumer | Home equity loans
Loans
Total loans on nonaccrual status	13,003,000		13,003,000	4,640,000
Non-accrual with no allowance	5,596,000		5,596,000
Consumer | Real estate
Loans
Total loans on nonaccrual status	50,884,000		50,884,000	14,358,000
Greater than 90 Days Accruing	310,000		310,000
Non-accrual with no allowance	25,820,000		25,820,000
Consumer | Consumer
Loans
Total loans on nonaccrual status	6,571,000		6,571,000	$ 3,191,000
Non-accrual with no allowance	$ 1,072,000		$ 1,072,000