Fair Value Measurements, Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Investment securities available for sale:
Investment securities available for sale	$ 4,300,490	$ 4,680,226
Trading assets	49,306	52,398
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	47,561	51,103
Fair Value, Inputs, Level 1
Investment securities available for sale:
Investment securities available for sale	2,606	2,602
Fair Value, Inputs, Level 2
Investment securities available for sale:
Investment securities available for sale	4,297,884	4,677,424
Trading assets	51,596	54,974
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	49,851	53,679
Fair Value, Inputs, Level 3
Investment securities available for sale:
Investment securities available for sale	0	200
Trading assets	1,803	3,114
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	373	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | US Treasury Securities
Investment securities available for sale:
Investment securities available for sale	1,005	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	1,601	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	2,606	2,602
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | FNMA / FHLMC
Investment securities available for sale:
Investment securities available for sale	590,419	639,930
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | GNMA
Investment securities available for sale:
Investment securities available for sale	2,039,826	2,004,475
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Private Label
Investment securities available for sale:
Investment securities available for sale	1,102	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	1,663,337	2,028,898
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	3,200	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	4,297,884	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	34,051	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Interest rate-related instruments — customer and mirror
Investment securities available for sale:
Trading assets	31,841	33,671
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	31,113	33,188
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign currency exchange forwards
Investment securities available for sale:
Trading assets	812	2,002
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	774	1,943
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commodity contracts
Investment securities available for sale:
Trading assets	16,653	16,725
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	15,674	15,972
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Purchased options (time deposit)
Investment securities available for sale:
Trading assets	2,290	2,576
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	2,290	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	1,803	206
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Forward commitments (mortgage)
Investment securities available for sale:
Trading assets	0	2,908
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	$ 373	$ 0