Fair Value Measurements - Fair value of balance sheet items (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets:
Investment securities available for sale	$ 5,765,192	$ 7,235,305
Derivative asset	877,940	354,982
Liabilities:
Derivative liabilities	247,266	135,900
U.S. Treasury and other U.S. government agencies
Assets:
Investment securities available for sale	2,217,738	3,127,525
Collateralized mortgage obligations
Assets:
Investment securities available for sale	2,449,995	2,781,125
State and political subdivisions
Assets:
Investment securities available for sale	714	798
Fair Value, measurements, recurring
Assets:
Trading account assets	1,016,966	473,976
Investment securities available for sale	5,765,192	7,235,305
Loans held for sale	245,851	112,058
Derivative asset	37,598	18,643
Liabilities:
Trading account liabilities	227,669	118,559
Derivative liabilities	14,750	11,437
Fair Value, measurements, recurring \| Interest rate contracts
Assets:
Derivative asset	34,207	13,907
Liabilities:
Derivative liabilities	8,584	3,732
Fair Value, measurements, recurring \| Equity contracts
Assets:
Derivative asset	1,868	4,460
Liabilities:
Derivative liabilities	1,555	3,765
Fair Value, measurements, recurring \| Foreign exchange contracts
Assets:
Derivative asset	1,523	276
Liabilities:
Derivative liabilities	4,611	3,940
Fair Value, measurements, recurring \| U.S. Treasury and other U.S. government agencies
Assets:
Trading account assets	176,614	137,637
Investment securities available for sale	2,217,738	3,127,525
Liabilities:
Trading account liabilities	1,619
Fair Value, measurements, recurring \| Interest rate contracts
Assets:
Trading account assets	789,245	313,573
Liabilities:
Trading account liabilities	177,644	97,881
Fair Value, measurements, recurring \| Foreign exchange contracts
Assets:
Trading account assets	51,097	22,766
Liabilities:
Trading account liabilities	48,406	20,678
Fair Value, measurements, recurring \| Mortgage-backed securities
Assets:
Investment securities available for sale	1,096,745	1,325,857
Fair Value, measurements, recurring \| Collateralized mortgage obligations
Assets:
Investment securities available for sale	2,449,995	2,781,125
Fair Value, measurements, recurring \| State and political subdivisions
Assets:
Trading account assets	10
Investment securities available for sale	714	798
Fair Value, measurements, recurring \| Equity securities
Assets:
Other assets	18,890	19,038
Fair Value, measurements, recurring \| Quoted Prices in Active Markets for Identical Assets, Level 1
Assets:
Trading account assets	176,614	137,637
Investment securities available for sale	1,730,483	2,598,471
Derivative asset	0	38
Liabilities:
Trading account liabilities	1,619	0
Fair Value, measurements, recurring \| Quoted Prices in Active Markets for Identical Assets, Level 1 \| Interest rate contracts
Assets:
Derivative asset	0	38
Fair Value, measurements, recurring \| Quoted Prices in Active Markets for Identical Assets, Level 1 \| U.S. Treasury and other U.S. government agencies
Assets:
Trading account assets	176,614	137,637
Investment securities available for sale	1,730,483	2,598,471
Liabilities:
Trading account liabilities	1,619
Fair Value, measurements, recurring \| Quoted Prices in Active Markets for Identical Assets, Level 1 \| Equity securities
Assets:
Other assets	18,890	19,038
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2
Assets:
Trading account assets	840,352	336,339
Investment securities available for sale	4,034,709	4,636,834
Loans held for sale	245,851	112,058
Derivative asset	16,855	15,517
Liabilities:
Trading account liabilities	226,050	118,559
Derivative liabilities	14,750	11,437
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Interest rate contracts
Assets:
Derivative asset	13,464	10,781
Liabilities:
Derivative liabilities	8,584	3,732
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Equity contracts
Assets:
Derivative asset	1,868	4,460
Liabilities:
Derivative liabilities	1,555	3,765
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Foreign exchange contracts
Assets:
Derivative asset	1,523	276
Liabilities:
Derivative liabilities	4,611	3,940
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| U.S. Treasury and other U.S. government agencies
Assets:
Trading account assets	0
Investment securities available for sale	487,255	529,054
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Interest rate contracts
Assets:
Trading account assets	789,245	313,573
Liabilities:
Trading account liabilities	177,644	97,881
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Foreign exchange contracts
Assets:
Trading account assets	51,097	22,766
Liabilities:
Trading account liabilities	48,406	20,678
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Mortgage-backed securities
Assets:
Investment securities available for sale	1,096,745	1,325,857
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Collateralized mortgage obligations
Assets:
Investment securities available for sale	2,449,995	2,781,125
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| State and political subdivisions
Assets:
Trading account assets	10
Investment securities available for sale	714	798
Fair Value, measurements, recurring \| Fair Value, Inputs, Level 3
Assets:
Trading account assets	0	0
Investment securities available for sale	0	0
Derivative asset	20,743	3,088
Liabilities:
Derivative liabilities	0	0
Fair Value, measurements, recurring \| Fair Value, Inputs, Level 3 \| Interest rate contracts
Assets:
Derivative asset	20,743	3,088
Liabilities:
Derivative liabilities	0	0
Fair Value, measurements, recurring \| Fair Value, Inputs, Level 3 \| Interest rate contracts
Assets:
Derivative asset	20,743	3,088
Other assets, MSR investments \| Fair Value, measurements, recurring
Assets:
Other assets	29,035	42,022
Other assets, MSR investments \| Fair Value, measurements, recurring \| Fair Value, Inputs, Level 3
Assets:
Other assets	29,035	42,022
Other assets - SBIC investments \| Fair Value, measurements, recurring
Assets:
Other assets	152,784	119,475
Other assets - SBIC investments \| Fair Value, measurements, recurring \| Fair Value, Inputs, Level 3
Assets:
Other assets	$ 152,784	$ 119,475

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Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

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Fair value portion of other assets.

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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