Fair Value Measurements, Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Investment securities available for sale:
Investment securities available for sale	$ 4,967,414	$ 5,396,812
Trading assets	32,192	35,163
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	33,430	37,329
Fair Value, Measurements, Recurring [Member]
Investment securities available for sale:
Trading assets	36,268	43,164
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	36,145	45,818
Fair Value, Measurements, Recurring [Member] | US Treasury Securities [Member]
Investment securities available for sale:
Investment securities available for sale	997	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		582,679
Fair Value, Measurements, Recurring [Member] | Government-Sponsored Enterprise (GSE) [Member]
Investment securities available for sale:
Investment securities available for sale	1,414,626	3,563,457
Fair Value, Measurements, Recurring [Member] | GNMA [Member]
Investment securities available for sale:
Investment securities available for sale	1,590,003	167,332
Fair Value, Measurements, Recurring [Member] | Private Label [Member]
Investment securities available for sale:
Investment securities available for sale	1,709	2,294
Fair Value, Measurements, Recurring [Member] | GNMA Commercial Mortgage-Related Securities [Member]
Investment securities available for sale:
Investment securities available for sale	1,955,310	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	4,769	6,159
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Investment securities available for sale:
Investment securities available for sale	2,566	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	997	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		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	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | GNMA [Member]
Investment securities available for sale:
Investment securities available for sale	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	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	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	1,569	2,959
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Investment securities available for sale:
Investment securities available for sale	4,964,648	5,392,655
Trading assets	34,907	41,217
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	36,145	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	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		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	1,414,626	3,563,457
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | GNMA [Member]
Investment securities available for sale:
Investment securities available for sale	1,590,003	167,332
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Private Label [Member]
Investment securities available for sale:
Investment securities available for sale	1,709	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	1,955,310	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	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	200	200
Trading assets	1,361	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	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		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	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | GNMA [Member]
Investment securities available for sale:
Investment securities available for sale	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	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	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	$ 200	$ 200