Investment Securities AFS and HTM Securities Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Investment securities available for sale:
Amortized cost	$ 5,344,040	$ 5,368,259
Gross unrealized gains	81,042	85,085
Gross unrealized losses	(21,426)	(56,532)
Fair value	5,403,656	5,396,812
Investment securities held to maturity:
Amortized cost	604,799	404,455
Gross unrealized gains	10,023	9,444
Gross unrealized losses	(1,450)	(832)
Fair value	613,372	413,067
US Treasury Securities [Member]
Investment securities available for sale:
Amortized cost	999	999
Gross unrealized gains	3	0
Gross unrealized losses	0	(1)
Fair value	1,002	998
Obligations of state and political subdivisions (municipal securities) [Member]
Investment securities available for sale:
Amortized cost	409,529	560,839
Gross unrealized gains	18,176	21,869
Gross unrealized losses	(4)	(29)
Fair value	427,701	582,679
Investment securities held to maturity:
Amortized cost	604,799	404,455
Gross unrealized gains	10,023	9,444
Gross unrealized losses	(1,450)	(832)
Fair value	613,372	413,067
Federal National Mortgage Association Certificates and Obligations (FNMA) and Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member]
Investment securities available for sale:
Amortized cost	1,932,740	3,534,240
Gross unrealized gains	52,232	59,640
Gross unrealized losses	(4,182)	(30,423)
Fair value	1,980,790	3,563,457
Government National Mortgage Association (GNMA) [Member]
Investment securities available for sale:
Amortized cost	1,106,601	165,863
Gross unrealized gains	4,106	1,596
Gross unrealized losses	(2,574)	(127)
Fair value	1,108,133	167,332
Private Label [Member]
Investment securities available for sale:
Amortized cost	1,755	2,297
Gross unrealized gains	1	7
Gross unrealized losses	(10)	(10)
Fair value	1,746	2,294
GSE commercial mortgage-related securities [Member]
Investment securities available for sale:
Amortized cost	1,885,776	1,097,913
Gross unrealized gains	6,461	1,922
Gross unrealized losses	(14,656)	(25,942)
Fair value	1,877,581	1,073,893
Other Debt And Other Equity Securities [Member]
Investment securities available for sale:
Amortized cost	6,640	6,108
Gross unrealized gains	63	51
Gross unrealized losses	0	0
Fair value	$ 6,703	$ 6,159

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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