Basis of presentation (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended
	Jan. 01, 2020	Mar. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			$ 91,459	$ 81,307	$ 53,355	$ 53,040	$ 58,425		$ 52,119
Allowance for loan commitments			7,600	8,100	1,741
Retained earnings	$ 622,042		645,943		622,042
Loans held for investment, net	5,067,821		5,389,443		5,067,821
Total assets	13,745,251		14,542,361		13,745,251
Deferred income taxes	379,324		376,356		379,324
Other	583,545		586,853		583,545
Total liabilities	11,430,698		12,184,837		11,430,698
Total shareholders’ equity	2,280,260		2,323,231	$ 2,291,398	2,280,260	$ 2,242,689	$ 2,208,586	$ 2,183,515	$ 2,162,280
Total liabilities and shareholders' equity	$ 13,745,251		$ 14,542,361		13,745,251
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member
As reported under ASU No. 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 606,670
Loans held for investment, net	5,048,380
Total assets	13,725,810
Deferred income taxes	373,696
Other	585,104
Total liabilities	11,426,629
Total shareholders’ equity	2,264,888	$ 2,276,505			2,264,888
Total liabilities and shareholders' equity	13,725,810
Impact of ASU No. 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss	21,000				19,441
Allowance for loan commitments	2,000				1,559
Retained earnings	(15,372)
Loans held for investment, net	(19,441)
Total assets	(19,441)
Deferred income taxes	(5,628)
Other	1,559
Total liabilities	(4,069)
Total shareholders’ equity	(15,372)				$ (15,372)
Total liabilities and shareholders' equity	$ (19,441)