Basis of presentation (Details) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
	Jan. 01, 2020	Jun. 30, 2020	Dec. 31, 2019	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss		$ 81,307	$ 53,355	$ 77,084	$ 58,425	$ 54,297	$ 52,119
Allowance for loan commitments		8,100	1,741	3,800
Retained earnings	$ 622,042	616,941	622,042
Loans held for investment, net	5,067,821	5,356,510	5,067,821
Total assets	13,745,251	14,480,700	13,745,251
Deferred income taxes	379,324	368,834	379,324
Other	583,545	580,082	583,545
Total liabilities	11,430,698	12,155,009	11,430,698
Total shareholders’ equity	2,280,260	2,291,398	2,280,260		$ 2,208,586	$ 2,183,515	$ 2,162,280
Total liabilities and shareholders' equity	$ 13,745,251	$ 14,480,700	$ 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		$ 2,264,888	$ 2,276,505
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)