Fair Value Measurements, Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	$ 4,680,226	$ 4,967,414
Trading assets	52,398	32,192
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	51,103	33,430
Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	2,602	2,566
Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	4,677,424	4,964,648
Trading assets	54,974	34,907
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	53,679	36,145
Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	200	200
Trading assets	3,114	1,361
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 | U. S. Treasury securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	1,000	997
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	1,602	1,569
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	2,602	2,566
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | FNMA / FHLMC
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	639,930	1,414,626
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | GNMA
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	2,004,475	1,590,003
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | Private-label
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	1,121	1,709
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	2,028,898	1,955,310
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	3,000	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	4,677,424	4,964,648
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | Interest rate-related instruments
Assets, Fair Value Disclosure [Abstract]
Trading assets	33,671	29,391
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	33,188	30,886
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | Foreign currency exchange forwards
Assets, Fair Value Disclosure [Abstract]
Trading assets	2,002	1,532
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	1,943	1,398
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading assets	16,725	1,269
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	15,972	1,146
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 | Purchased options (time deposit)
Assets, Fair Value Disclosure [Abstract]
Trading assets	2,576	2,715
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	2,576	2,715
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	200	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	200	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	206	958
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 | Forward commitments to sell residential mortgage loans
Assets, Fair Value Disclosure [Abstract]
Trading assets	$ 2,908	$ 403