Fair Value Measurements - Fair value of balance sheet items (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets:
Investment securities available for sale	$ 6,028,072	$ 7,235,305
Derivative asset	782,120	354,982
Liabilities:
Derivative liabilities	206,043	135,900
U.S. Treasury and other U.S. government agencies
Assets:
Investment securities available for sale	2,191,009	3,127,525
Collateralized mortgage obligations
Assets:
Investment securities available for sale	2,866,505	2,781,125
State and political subdivisions
Assets:
Investment securities available for sale	636	798
Fair Value, measurements, recurring
Assets:
Trading account assets	926,497	473,976
Investment securities available for sale	6,028,072	7,235,305
Loans held for sale	253,454	112,058
Derivative asset	32,803	18,643
Liabilities:
Trading account liabilities	191,182	118,559
Derivative liabilities	8,426	11,437
Fair Value, measurements, recurring \| Interest rate contracts
Assets:
Derivative asset	29,351	13,907
Liabilities:
Derivative liabilities	5,830	3,732
Fair Value, measurements, recurring \| Equity contracts
Assets:
Derivative asset	774	4,460
Liabilities:
Derivative liabilities	625	3,765
Fair Value, measurements, recurring \| Foreign exchange contracts
Assets:
Derivative asset	2,678	276
Liabilities:
Derivative liabilities	1,971	3,940
Fair Value, measurements, recurring \| U.S. Treasury and other U.S. government agencies
Assets:
Trading account assets	177,180	137,637
Investment securities available for sale	2,191,009	3,127,525
Fair Value, measurements, recurring \| Interest rate contracts
Assets:
Trading account assets	711,389	313,573
Liabilities:
Trading account liabilities	155,542	97,881
Fair Value, measurements, recurring \| Foreign exchange contracts
Assets:
Trading account assets	37,928	22,766
Liabilities:
Trading account liabilities	35,640	20,678
Fair Value, measurements, recurring \| Mortgage-backed securities
Assets:
Investment securities available for sale	969,922	1,325,857
Fair Value, measurements, recurring \| Collateralized mortgage obligations
Assets:
Investment securities available for sale	2,866,505	2,781,125
Fair Value, measurements, recurring \| State and political subdivisions
Assets:
Investment securities available for sale	636	798
Fair Value, measurements, recurring \| Equity securities
Assets:
Other assets	28,035	19,038
Fair Value, measurements, recurring \| Quoted Prices in Active Markets for Identical Assets, Level 1
Assets:
Trading account assets	177,180	137,637
Investment securities available for sale	1,725,371	2,598,471
Derivative asset	0	38
Liabilities:
Trading account liabilities	0	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	177,180	137,637
Investment securities available for sale	1,725,371	2,598,471
Fair Value, measurements, recurring \| Quoted Prices in Active Markets for Identical Assets, Level 1 \| Equity securities
Assets:
Other assets	28,035	19,038
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2
Assets:
Trading account assets	749,317	336,339
Investment securities available for sale	4,302,701	4,636,834
Loans held for sale	253,454	112,058
Derivative asset	16,498	15,517
Liabilities:
Trading account liabilities	191,182	118,559
Derivative liabilities	8,426	11,437
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Interest rate contracts
Assets:
Derivative asset	13,046	10,781
Liabilities:
Derivative liabilities	5,830	3,732
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Equity contracts
Assets:
Derivative asset	774	4,460
Liabilities:
Derivative liabilities	625	3,765
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Foreign exchange contracts
Assets:
Derivative asset	2,678	276
Liabilities:
Derivative liabilities	1,971	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	465,638	529,054
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Interest rate contracts
Assets:
Trading account assets	711,389	313,573
Liabilities:
Trading account liabilities	155,542	97,881
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Foreign exchange contracts
Assets:
Trading account assets	37,928	22,766
Liabilities:
Trading account liabilities	35,640	20,678
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Mortgage-backed securities
Assets:
Investment securities available for sale	969,922	1,325,857
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Collateralized mortgage obligations
Assets:
Investment securities available for sale	2,866,505	2,781,125
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| State and political subdivisions
Assets:
Investment securities available for sale	636	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	16,305	3,088
Liabilities:
Derivative liabilities	0	0
Fair Value, measurements, recurring \| Fair Value, Inputs, Level 3 \| Interest rate contracts
Assets:
Derivative asset	16,305	3,088
Liabilities:
Derivative liabilities	0	0
Fair Value, measurements, recurring \| Fair Value, Inputs, Level 3 \| Interest rate contracts
Assets:
Derivative asset	16,305	3,088
Other assets, MSR investments \| Fair Value, measurements, recurring
Assets:
Other assets	28,731	42,022
Other assets, MSR investments \| Fair Value, measurements, recurring \| Fair Value, Inputs, Level 3
Assets:
Other assets	28,731	42,022
Other assets - SBIC investments \| Fair Value, measurements, recurring
Assets:
Other assets	164,362	119,475
Other assets - SBIC investments \| Fair Value, measurements, recurring \| Fair Value, Inputs, Level 3
Assets:
Other assets	$ 164,362	$ 119,475

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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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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