Fair Value Measurements, Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	$ 4,043,446	$ 4,680,226
Trading Assets	69,675	52,398
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	67,660	51,103
Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	2,628	2,602
Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	4,040,818	4,677,424
Trading Assets	70,850	54,974
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	68,835	53,679
Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	0	200
Trading Assets	1,538	3,114
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	313	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 | U.S. Treasury securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	996	1,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 | Other securities (debt and equity)
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	1,632	1,602
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 | Total investment securities available for sale
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	2,628	2,602
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | FNMA / FHLMC
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	464,768	639,930
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | GNMA
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	1,913,350	2,004,475
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | Private-label
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	1,059	1,121
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | GNMA commercial mortgage-related securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	1,513,277	2,028,898
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | Federal Family Education Loan Program (FFELP) Guaranteed Loans
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	145,176	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | Other securities (debt and equity)
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	3,188	3,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | Total investment securities available for sale
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	4,040,818	4,677,424
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | Residential Loans Held For Sale Member [Member]
Assets, Fair Value Disclosure [Abstract]
Trading Assets	85,544	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | Interest rate-related instruments
Assets, Fair Value Disclosure [Abstract]
Trading Assets	28,494	33,671
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	28,035	33,188
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | Foreign currency exchange forwards
Assets, Fair Value Disclosure [Abstract]
Trading Assets	2,495	2,002
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	2,339	1,943
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading Assets	38,686	16,725
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	37,286	15,972
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | Purchased options (time deposit)
Assets, Fair Value Disclosure [Abstract]
Trading Assets	1,175	2,576
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	1,175	2,576
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 | Other securities (debt and equity)
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	0	200
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 | Total investment securities available for sale
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	0	200
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 | Interest rate lock commitments to originate residential mortgage loans held for sale
Assets, Fair Value Disclosure [Abstract]
Trading Assets	1,538	206
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 | Forward commitments to sell residential mortgage loans
Assets, Fair Value Disclosure [Abstract]
Trading Assets	0	2,908
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	$ 313	$ 0