Fair Value Measurements, Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investment securities available for sale:
Investment securities available for sale	$ 4,261,651	$ 4,043,446
Trading assets	132,919	69,675
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	131,612	67,660
Fair Value, Inputs, Level 1
Investment securities available for sale:
Investment securities available for sale	2,609	2,628
Fair Value, Inputs, Level 2
Investment securities available for sale:
Investment securities available for sale	4,259,042	4,040,818
Trading assets	136,342	70,850
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	132,639	68,835
Fair Value, Inputs, Level 3
Investment securities available for sale:
Trading assets	3,287	1,538
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	919	313
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | US Treasury Securities
Investment securities available for sale:
Investment securities available for sale	996	996
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Other Debt And Other Equity Securities
Investment securities available for sale:
Investment securities available for sale	1,613	1,632
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Total investment securities available for sale
Investment securities available for sale:
Investment securities available for sale	2,609	2,628
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | FNMA / FHLMC
Investment securities available for sale:
Investment securities available for sale	396,709	464,768
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | GNMA
Investment securities available for sale:
Investment securities available for sale	2,210,189	1,913,350
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Private Label
Investment securities available for sale:
Investment securities available for sale	1,031	1,059
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | GNMA Commercial Mortgage-Related Securities
Investment securities available for sale:
Investment securities available for sale	1,348,282	1,513,277
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Investment securities available for sale:
Investment securities available for sale	299,831	145,176
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Debt And Other Equity Securities
Investment securities available for sale:
Investment securities available for sale	3,000	3,188
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Total investment securities available for sale
Investment securities available for sale:
Investment securities available for sale	4,259,042	4,040,818
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Residential Loans Held For Sale Member
Investment securities available for sale:
Trading assets	143,022	85,544
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Interest rate-related instruments — customer and mirror
Investment securities available for sale:
Trading assets	62,708	28,494
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	62,141	28,035
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign currency exchange forwards
Investment securities available for sale:
Trading assets	3,317	2,495
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	3,091	2,339
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commodity contracts
Investment securities available for sale:
Trading assets	66,894	38,686
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	66,380	37,286
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Purchased options (time deposit)
Investment securities available for sale:
Trading assets	1,027	1,175
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	1,027	1,175
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Interest rate lock commitments (mortgage)
Investment securities available for sale:
Trading assets	3,287	1,538
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Forward commitments (mortgage)
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	919	313
Designated as Hedging Instrument
Investment securities available for sale:
Trading assets	500,000
Designated as Hedging Instrument | Fair Value, Inputs, Level 2
Investment securities available for sale:
Interest Rate Fair Value Hedge Derivative at Fair Value, Net	$ 2,396	$ 0