Basis of presentation (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss	$ 77,084		$ 53,355	$ 54,297	$ 52,119
Allowance for loan commitments	3,800		1,741
Retained earnings	604,071	$ 622,042	622,042
Total shareholders’ equity	2,276,505	2,280,260	2,280,260	$ 2,183,515	$ 2,162,280
Loans held for investment, net	5,103,848	5,067,821	5,067,821
Total assets	13,849,018	13,745,251	13,745,251
Deferred income taxes	382,872	379,324	379,324
Other	521,978	583,545	583,545
Total liabilities	11,538,220	11,430,698	11,430,698
Total liabilities and shareholders' equity	$ 13,849,018	13,745,251	13,745,251
As reported under ASU No. 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		606,670
Total shareholders’ equity		2,264,888
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 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)
Total shareholders’ equity		(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 liabilities and shareholders' equity		$ (19,441)