Fair Value Measurements Asset and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Investment securities available for sale:
Investment securities available for sale, at fair value	$ 5,407,998	$ 5,396,812
Trading assets	35,386	35,163
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	37,169	37,329
Fair Value, Measurements, Recurring [Member]
Investment securities available for sale:
Trading assets	44,395	43,164
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	42,332	45,818
Fair Value, Measurements, Recurring [Member] | US Treasury Securities [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	1,001	998
Fair Value, Measurements, Recurring [Member] | Obligations of state and political subdivisions (municipal securities) [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	470,103	582,679
Fair Value, Measurements, Recurring [Member] | Government National Mortgage Association (GNMA) [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	3,153,711	3,730,789
Fair Value, Measurements, Recurring [Member] | Private Label [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	1,917	2,294
Fair Value, Measurements, Recurring [Member] | GNMA commercial mortgage-related securities [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	1,774,562	1,073,893
Fair Value, Measurements, Recurring [Member] | Other Debt And Other Equity Securities [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	6,704	6,159
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	4,505	3,957
Trading assets	0	0
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	1,001	998
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Obligations of state and political subdivisions (municipal securities) [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Government National Mortgage Association (GNMA) [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Private Label [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | GNMA commercial mortgage-related securities [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Debt And Other Equity Securities [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	3,504	2,959
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	5,403,293	5,392,655
Trading assets	40,549	41,217
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	42,332	43,383
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Obligations of state and political subdivisions (municipal securities) [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	470,103	582,679
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Government National Mortgage Association (GNMA) [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	3,153,711	3,730,789
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Private Label [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	1,917	2,294
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | GNMA commercial mortgage-related securities [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	1,774,562	1,073,893
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Debt And Other Equity Securities [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	3,000	3,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	200	200
Trading assets	3,846	1,947
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	0	2,435
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | US Treasury Securities [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Obligations of state and political subdivisions (municipal securities) [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Government National Mortgage Association (GNMA) [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Private Label [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | GNMA commercial mortgage-related securities [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Debt And Other Equity Securities [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	$ 200	$ 200