Fair Value Measurements - Fair value of balance sheet items (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Trading account assets	$ 216,465	$ 220,496
Investment securities available for sale	11,434,152	12,219,632
Derivative asset	178,769	208,094
Other assets	2,109,484	1,982,648
Liabilities:
Derivative liabilities	250,426	207,932
Fair Value, measurements, recurring
Assets:
Trading account assets	216,465	220,496
Investment securities available for sale	11,434,152	12,219,632
Loans held for sale	76,401	67,110
Derivative asset	54,750	62,429
Liabilities:
Trading account liabilities	204,698	162,593
Derivative liabilities	73,109	60,839
Fair Value, measurements, recurring | Interest rate contracts
Assets:
Derivative asset	18,142	22,263
Liabilities:
Derivative liabilities	41,885	21,387
Fair Value, measurements, recurring | Equity contracts
Assets:
Derivative asset	34,248	39,791
Liabilities:
Derivative liabilities	30,435	35,562
Fair Value, measurements, recurring | Foreign exchange contracts
Assets:
Derivative asset	2,360	375
Liabilities:
Derivative liabilities	789	3,890
Fair Value, measurements, recurring | U.S. Treasury and other U.S. government agencies
Assets:
Trading account assets	91,770	74,195
Investment securities available for sale	4,794,332	4,204,438
Liabilities:
Trading account liabilities	29,999	17,996
Fair Value, measurements, recurring | State and political subdivisions
Assets:
Trading account assets	561	557
Investment securities available for sale	1,653	2,383
Fair Value, measurements, recurring | Collateralized mortgage obligations
Assets:
Investment securities available for sale	4,016,289	5,200,011
Fair Value, measurements, recurring | Other debt securities
Assets:
Trading account assets	115	79
Fair Value, measurements, recurring | Interest rate contracts
Assets:
Trading account assets	110,694	133,516
Liabilities:
Trading account liabilities	163,060	134,073
Fair Value, measurements, recurring | Foreign exchange contracts
Assets:
Trading account assets	13,325	12,149
Liabilities:
Trading account liabilities	11,639	10,524
Fair Value, measurements, recurring | Mortgage-backed securities
Assets:
Investment securities available for sale	2,621,878	2,812,800
Fair Value, measurements, recurring | Equity Securities
Assets:
Other assets	18,610	13,577
Fair Value, measurements, recurring | Quoted Prices in Active Markets for Identical Assets, Level 1
Assets:
Trading account assets	91,770	74,195
Investment securities available for sale	3,906,091	3,248,898
Derivative asset	0	38
Liabilities:
Trading account liabilities	29,999	17,996
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	91,770	74,195
Investment securities available for sale	3,906,091	3,248,898
Liabilities:
Trading account liabilities	29,999	17,996
Fair Value, measurements, recurring | Quoted Prices in Active Markets for Identical Assets, Level 1 | Equity Securities
Assets:
Other assets	18,610	13,577
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2
Assets:
Trading account assets	124,695	146,301
Investment securities available for sale	7,528,061	8,970,734
Loans held for sale	76,401	67,110
Derivative asset	52,141	59,975
Liabilities:
Trading account liabilities	174,699	144,597
Derivative liabilities	73,108	60,839
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | Interest rate contracts
Assets:
Derivative asset	15,533	19,809
Liabilities:
Derivative liabilities	41,884	21,387
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | Equity contracts
Assets:
Derivative asset	34,248	39,791
Liabilities:
Derivative liabilities	30,435	35,562
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | Foreign exchange contracts
Assets:
Derivative asset	2,360	375
Liabilities:
Derivative liabilities	789	3,890
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | U.S. Treasury and other U.S. government agencies
Assets:
Investment securities available for sale	888,241	955,540
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | State and political subdivisions
Assets:
Trading account assets	561	557
Investment securities available for sale	1,653	2,383
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | Collateralized mortgage obligations
Assets:
Investment securities available for sale	4,016,289	5,200,011
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | Other debt securities
Assets:
Trading account assets	115	79
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | Interest rate contracts
Assets:
Trading account assets	110,694	133,516
Liabilities:
Trading account liabilities	163,060	134,073
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | Foreign exchange contracts
Assets:
Trading account assets	13,325	12,149
Liabilities:
Trading account liabilities	11,639	10,524
Fair Value, measurements, recurring | Significant Other Observable Inputs, Level 2 | Mortgage-backed securities
Assets:
Investment securities available for sale	2,621,878	2,812,800
Fair Value, measurements, recurring | Fair Value, Inputs, Level 3
Assets:
Trading account assets	0	0
Investment securities available for sale	0	0
Derivative asset	2,609	2,416
Liabilities:
Derivative liabilities	1	0
Fair Value, measurements, recurring | Fair Value, Inputs, Level 3 | Interest rate contracts
Assets:
Derivative asset	2,609	2,416
Liabilities:
Derivative liabilities	1	0
Other assets, MSR | Fair Value, measurements, recurring
Assets:
Other assets	53,025	49,597
Other assets, MSR | Fair Value, measurements, recurring | Fair Value, Inputs, Level 3
Assets:
Other assets	53,025	49,597
Other assets - SBIC investments | Fair Value, measurements, recurring
Assets:
Other assets	47,987	45,042
Other assets - SBIC investments | Fair Value, measurements, recurring | Fair Value, Inputs, Level 3
Assets:
Other assets	$ 47,987	$ 45,042