Fair Value Measurements Asset and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Investment securities available for sale:
Investment securities available for sale, at fair value	$ 5,403,656	$ 5,396,812
Trading assets	43,752	35,163
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	45,817	37,329
Fair Value, Measurements, Recurring [Member]
Investment securities available for sale:
Trading assets	49,251	43,164
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	50,714	45,818
Fair Value, Measurements, Recurring [Member] | US Treasury Securities [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	1,002	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	427,701	582,679
Fair Value, Measurements, Recurring [Member] | Government Sponsored Enterprise (GSE) [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	3,088,923	3,730,789
Fair Value, Measurements, Recurring [Member] | Private Label [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	1,746	2,294
Fair Value, Measurements, Recurring [Member] | GSE commercial mortgage-related securities [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	1,877,581	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	6,703	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	1,002	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 Sponsored Enterprise (GSE) [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] | GSE 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	3,503	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	5,398,951	5,392,655
Trading assets	46,667	41,217
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	48,732	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	427,701	582,679
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Government Sponsored Enterprise (GSE) [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	3,088,923	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	1,746	2,294
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | GSE commercial mortgage-related securities [Member]
Investment securities available for sale:
Investment securities available for sale, at fair value	1,877,581	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	2,584	1,947
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	1,982	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 Sponsored Enterprise (GSE) [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] | GSE 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