Fair Value Measurements, Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	$ 3,258,360	$ 3,262,586
Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	26,650	0
Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	3,231,711	3,262,586
Residential Held-for-sale, Fair Value Disclosure	130,139	136,280
Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure [Abstract]
Trading assets	7,888	2,527
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	0	710
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Other Debt And Other Equity Securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	1,646	1,646
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Total investment securities available for sale
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	26,650	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | GNMA
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	407,313	985,139
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | FFELP
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	325,843	263,693
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Debt And Other Equity Securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	2,997	3,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Total investment securities available for sale
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	3,231,711	3,262,586
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Residential loans held for sale
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	130,139	136,280
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Interest rate-related instruments — customer and mirror, net
Assets, Fair Value Disclosure [Abstract]
Trading assets	225,912	77,024
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	28,960	13,073
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign currency exchange forwards
Assets, Fair Value Disclosure [Abstract]
Trading assets	4,710	4,226
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	4,563	4,048
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading assets	25,935	20,528
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	24,476	19,624
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Mortgage banking(a)(b)
Assets, Fair Value Disclosure [Abstract]
Trading assets	7,888	2,527
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Forward commitments (mortgage)
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	0	710
FNMA / FHLMC | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | FNMA / FHLMC
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	1,276,463	132,660
FNMA / FHLMC | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commercial mortgage-related securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	21,877	21,728
GNMA | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commercial mortgage-related securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	688,378	1,310,207
US Treasury Securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	26,650
US Treasury Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	26,650	0
Obligations of state and political subdivisions (municipal securities)
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	474,695	546,160
Obligations of state and political subdivisions (municipal securities) | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	474,695	546,160
Agency Securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	25,033
Agency Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	25,033	0
SBA
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	9,111
SBA | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	$ 9,111	$ 0