Fair Value Measurements - Fair value of balance sheet items (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets:
Investment securities available for sale	$ 6,344,816	$ 7,235,305
Derivative asset	879,744	354,982
Liabilities:
Derivative liabilities	270,910	135,900
U.S. Treasury and other U.S. government agencies
Assets:
Investment securities available for sale	2,570,313	3,127,525
Collateralized mortgage obligations
Assets:
Investment securities available for sale	2,588,110	2,781,125
State and political subdivisions
Assets:
Investment securities available for sale	717	798
Fair Value, measurements, recurring
Assets:
Trading account assets	1,009,130	473,976
Investment securities available for sale	6,344,816	7,235,305
Loans held for sale	117,752	112,058
Derivative asset	41,340	18,643
Liabilities:
Trading account liabilities	242,885	118,559
Derivative liabilities	22,519	11,437
Fair Value, measurements, recurring \| Interest rate contracts
Assets:
Derivative asset	31,191	13,907
Liabilities:
Derivative liabilities	17,133	3,732
Fair Value, measurements, recurring \| Equity contracts
Assets:
Derivative asset	2,572	4,460
Liabilities:
Derivative liabilities	2,141	3,765
Fair Value, measurements, recurring \| Foreign exchange contracts
Assets:
Derivative asset	7,577	276
Liabilities:
Derivative liabilities	3,245	3,940
Fair Value, measurements, recurring \| U.S. Treasury and other U.S. government agencies
Assets:
Trading account assets	170,378	137,637
Investment securities available for sale	2,570,313	3,127,525
Fair Value, measurements, recurring \| Interest rate contracts
Assets:
Trading account assets	780,757	313,573
Liabilities:
Trading account liabilities	187,995	97,881
Fair Value, measurements, recurring \| Foreign exchange contracts
Assets:
Trading account assets	57,647	22,766
Liabilities:
Trading account liabilities	54,890	20,678
Fair Value, measurements, recurring \| Mortgage-backed securities
Assets:
Investment securities available for sale	1,185,676	1,325,857
Fair Value, measurements, recurring \| Collateralized mortgage obligations
Assets:
Investment securities available for sale	2,588,110	2,781,125
Fair Value, measurements, recurring \| State and political subdivisions
Assets:
Trading account assets	348
Investment securities available for sale	717	798
Fair Value, measurements, recurring \| Equity securities
Assets:
Other assets	19,990	19,038
Fair Value, measurements, recurring \| Quoted Prices in Active Markets for Identical Assets, Level 1
Assets:
Trading account assets	170,378	137,637
Investment securities available for sale	2,091,237	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	170,378	137,637
Investment securities available for sale	2,091,237	2,598,471
Fair Value, measurements, recurring \| Quoted Prices in Active Markets for Identical Assets, Level 1 \| Equity securities
Assets:
Other assets	19,990	19,038
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2
Assets:
Trading account assets	838,752	336,339
Investment securities available for sale	4,253,579	4,636,834
Loans held for sale	117,752	112,058
Derivative asset	27,016	15,517
Liabilities:
Trading account liabilities	242,885	118,559
Derivative liabilities	20,605	11,437
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Interest rate contracts
Assets:
Derivative asset	16,867	10,781
Liabilities:
Derivative liabilities	15,219	3,732
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Equity contracts
Assets:
Derivative asset	2,572	4,460
Liabilities:
Derivative liabilities	2,141	3,765
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Foreign exchange contracts
Assets:
Derivative asset	7,577	276
Liabilities:
Derivative liabilities	3,245	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	479,076	529,054
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Interest rate contracts
Assets:
Trading account assets	780,757	313,573
Liabilities:
Trading account liabilities	187,995	97,881
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Foreign exchange contracts
Assets:
Trading account assets	57,647	22,766
Liabilities:
Trading account liabilities	54,890	20,678
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Mortgage-backed securities
Assets:
Investment securities available for sale	1,185,676	1,325,857
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| Collateralized mortgage obligations
Assets:
Investment securities available for sale	2,588,110	2,781,125
Fair Value, measurements, recurring \| Significant Other Observable Inputs, Level 2 \| State and political subdivisions
Assets:
Trading account assets	348
Investment securities available for sale	717	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	14,324	3,088
Liabilities:
Derivative liabilities	1,914	0
Fair Value, measurements, recurring \| Fair Value, Inputs, Level 3 \| Interest rate contracts
Assets:
Derivative asset	14,324	3,088
Liabilities:
Derivative liabilities	1,914	0
Fair Value, measurements, recurring \| Fair Value, Inputs, Level 3 \| Interest rate contracts
Assets:
Derivative asset	12,410	3,088
Other assets, MSR investments \| Fair Value, measurements, recurring
Assets:
Other assets	31,232	42,022
Other assets, MSR investments \| Fair Value, measurements, recurring \| Fair Value, Inputs, Level 3
Assets:
Other assets	31,232	42,022
Other assets - SBIC investments \| Fair Value, measurements, recurring
Assets:
Other assets	156,400	119,475
Other assets - SBIC investments \| Fair Value, measurements, recurring \| Fair Value, Inputs, Level 3
Assets:
Other assets	$ 156,400	$ 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, after the effects of master netting arrangements, of a financial liability or 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 liabilities 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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Fair value portion of liabilities pertaining to principal and customer trading transactions, or which may be incurred with the objective of generating a profit from short-term fluctuations in price as part of an entity's market-making, hedging and proprietary trading. Examples include, but are not limited to, short positions in securities, derivatives and commodities.

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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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