Fair Value Measurements, Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	$ 3,829,388	$ 3,946,941
Trading assets	77,920	88,943
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	77,859	87,668
Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	0	999
Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	3,829,388	3,945,943
Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure [Abstract]
Trading assets	3,032	2,208
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	1,825	2,072
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | US Treasury Securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	0	999
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	1,609	1,568
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	0	999
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | GNMA
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	2,038,100	2,128,531
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Private-label
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	878	1,003
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | FFELP asset backed securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	276,899	297,360
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	3,000	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	3,829,388	3,945,943
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Residential Loans Held For Sale Member
Assets, Fair Value Disclosure [Abstract]
Trading assets	81,392	64,321
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commercial loans held for sale [Domain]
Assets, Fair Value Disclosure [Abstract]
Trading assets	15,467	14,943
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Interest rate-related instruments — customer and mirror
Assets, Fair Value Disclosure [Abstract]
Trading assets	54,479	52,796
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	55,008	52,653
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign currency exchange forwards
Assets, Fair Value Disclosure [Abstract]
Trading assets	1,737	721
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	1,665	675
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading assets	21,705	35,426
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	21,186	34,340
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Purchased options (time deposit)
Assets, Fair Value Disclosure [Abstract]
Trading assets	156	109
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	156	109
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Interest rate lock commitments (mortgage)
Assets, Fair Value Disclosure [Abstract]
Trading assets	3,032	2,208
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Forward commitments (mortgage)
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	1,825	2,072
Designated as Hedging Instrument
Assets, Fair Value Disclosure [Abstract]
Trading assets	500,000
Designated as Hedging Instrument | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Interest Rate Fair Value Hedge Derivative at Fair Value, Net	128	40
FNMA/FHLMC | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | FNMA / FHLMC
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	242,074	295,252
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	20,655	0
GNMA | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commercial mortgage-related securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	$ 1,247,782	$ 1,220,797