Fair Value Measurements, Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment securities available for sale:
Investment securities available for sale	$ 4,485,875	$ 4,043,446
Trading assets	102,890	69,675
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	100,247	67,660
Fair Value, Inputs, Level 1
Investment securities available for sale:
Investment securities available for sale	2,594	2,628
Fair Value, Inputs, Level 2
Investment securities available for sale:
Investment securities available for sale	4,483,281	4,040,818
Trading assets	104,099	70,850
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	101,198	68,835
Fair Value, Inputs, Level 3
Investment securities available for sale:
Trading assets	3,032	1,538
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	303	313
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | US Treasury Securities
Investment securities available for sale:
Investment securities available for sale	995	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	1,599	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	2,594	2,628
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | FNMA / FHLMC
Investment securities available for sale:
Investment securities available for sale	430,073	464,768
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | GNMA
Investment securities available for sale:
Investment securities available for sale	2,310,306	1,913,350
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Private Label
Investment securities available for sale:
Investment securities available for sale	1,044	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	1,448,369	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	290,300	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	3,188	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	4,483,281	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	103,953	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	53,189	28,494
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	52,238	28,035
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign currency exchange forwards
Investment securities available for sale:
Trading assets	2,612	2,495
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	2,484	2,339
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commodity contracts
Investment securities available for sale:
Trading assets	47,089	38,686
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	45,525	37,286
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Purchased options (time deposit)
Investment securities available for sale:
Trading assets	951	1,175
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	951	1,175
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Interest rate lock commitments (mortgage)
Investment securities available for sale:
Trading assets	3,032	1,538
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Forward commitments (mortgage)
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	303	313
Total commercial | Fair Value, Inputs, Level 2
Investment securities available for sale:
Loans Held-for-sale, Fair Value Disclosure	6,091	0
Designated as Hedging Instrument
Investment securities available for sale:
Trading assets	250,000
Designated as Hedging Instrument | Fair Value, Inputs, Level 2
Investment securities available for sale:
Interest Rate Fair Value Hedge Derivative at Fair Value, Net	$ 258	$ 0