Fair Value Measurements, Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Investment securities available for sale:
Investment securities available for sale	$ 3,801,699	$ 4,680,226
Trading assets	48,429	52,398
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	46,812	51,103
Fair Value, Inputs, Level 1
Investment securities available for sale:
Investment securities available for sale	2,651	2,602
Fair Value, Inputs, Level 2
Investment securities available for sale:
Investment securities available for sale	3,799,048	4,677,424
Trading assets	49,863	54,974
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	48,246	53,679
Fair Value, Inputs, Level 3
Investment securities available for sale:
Investment securities available for sale	0	200
Trading assets	2,963	3,114
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | US Treasury Securities
Investment securities available for sale:
Investment securities available for sale	1,001	1,000
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	1,650	1,602
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	2,651	2,602
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | FNMA / FHLMC
Investment securities available for sale:
Investment securities available for sale	505,261	639,930
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | GNMA
Investment securities available for sale:
Investment securities available for sale	1,667,836	2,004,475
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Private Label
Investment securities available for sale:
Investment securities available for sale	1,072	1,121
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | GNMA Commercial Mortgage-Related Securities
Investment securities available for sale:
Investment securities available for sale	1,513,077	2,028,898
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Asset backed Securities
Investment securities available for sale:
Investment securities available for sale	108,628	0
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	3,174	3,000
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	3,799,048	4,677,424
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Residential Loans Held For Sale Member
Investment securities available for sale:
Trading assets	113,064	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Interest rate-related instruments — customer and mirror
Investment securities available for sale:
Trading assets	26,837	33,671
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	26,469	33,188
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign currency exchange forwards
Investment securities available for sale:
Trading assets	3,053	2,002
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	3,033	1,943
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commodity contracts
Investment securities available for sale:
Trading assets	18,539	16,725
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	17,310	15,972
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Purchased options (time deposit)
Investment securities available for sale:
Trading assets	1,434	2,576
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	1,434	2,576
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Other Debt And Other Equity Securities
Investment securities available for sale:
Investment securities available for sale	0	200
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Total investment securities available for sale
Investment securities available for sale:
Investment securities available for sale	0	200
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Interest rate lock commitments (mortgage)
Investment securities available for sale:
Trading assets	2,729	206
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Forward commitments (mortgage)
Investment securities available for sale:
Trading assets	$ 234	$ 2,908