Segment reporting - Additional Information (Details) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments \| segment			2
Other noninterest expense	$ 36,710	$ 30,336	$ 109,447	$ 89,984
Interest income	67,639	57,904	194,363	163,126
Mortgage Segment
Segment Reporting Information [Line Items]
Other noninterest expense	112	0	1,995	0
Interest paid	$ 2,875	$ 3,997	$ 8,723	$ 13,022
Mortgage Segment \| Prime Interest Rate
Segment Reporting Information [Line Items]
Warehouse line of credit interest rate	5.00%	5.25%	5.00%	5.25%
Banking Segment
Segment Reporting Information [Line Items]
Other noninterest expense	$ 36,598	$ 30,336	$ 107,452	$ 89,984
Interest income	$ (5,848)	$ 0	$ 3,997	$ 13,022

The aggregate interest and fee income generated by: (1) loans the Entity has positive intent and ability to hold for the foreseeable future, or until maturity or payoff, including commercial and consumer loans, whether domestic or foreign, which may consist of: (a) industrial and agricultural; (b) real estate; and (c) real estate construction loans; (d) trade financing; (e) lease financing; (f) home equity lines-of-credit; (g) automobile and other vehicle loans; and (h) credit card and other revolving-type loans and (2) loans and leases held-for-sale which may include mortgage loans, direct financing, and sales-type leases.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.1)
-URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260

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+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4297-108586

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Section S99
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-URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260

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