Investment Securities, AFS and HTM Securities Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Investment securities available for sale:
Amortized Cost	$ 4,809,893	$ 4,984,867
Gross Unrealized Gains	48,454	35,684
Gross Unrealized Losses	(12,259)	(53,137)
Fair Value	4,846,088	4,967,414
Investment securities held to maturity:
Amortized Cost	1,253,494	1,168,230
Gross Unrealized Gains	34,609	17,505
Gross Unrealized Losses	(273)	(1,293)
Fair Value	1,287,830	1,184,442
U.S. Treasury securities
Investment securities available for sale:
Amortized Cost	1,000	999
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	(2)
Fair Value	1,001	997
Obligations of state and political subdivisions (municipal securities) [Member]
Investment securities held to maturity:
Amortized Cost	1,129,056	1,043,767
Gross Unrealized Gains	31,897	16,803
Gross Unrealized Losses	(233)	(339)
Fair Value	1,160,720	1,060,231
Private-label
Investment securities available for sale:
Amortized Cost	1,215	1,722
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(15)	(14)
Fair Value	1,200	1,709
Investment securities held to maturity:
Amortized Cost	0
Fair Value	0
GNMA Commercial Mortgage-Related Securities [Member]
Investment securities available for sale:
Amortized Cost	2,121,913	1,982,477
Gross Unrealized Gains	4,218	1,334
Gross Unrealized Losses	(11,462)	(28,501)
Fair Value	2,114,669	1,955,310
Investment securities held to maturity:
Amortized Cost	0
Fair Value	0
Other Debt And Other Equity Securities [Member]
Investment securities available for sale:
Amortized Cost	4,718	4,718
Gross Unrealized Gains	111	51
Gross Unrealized Losses	0	0
Fair Value	4,829	4,769
FNMA and FHLMC [Member] \| Residential Related Securities [Member]
Investment securities available for sale:
Amortized Cost	882,951	1,388,995
Gross Unrealized Gains	29,941	33,791
Gross Unrealized Losses	(32)	(8,160)
Fair Value	912,860	1,414,626
Investment securities held to maturity:
Amortized Cost	38,297	41,469
Gross Unrealized Gains	1,009	513
Gross Unrealized Losses	(16)	(645)
Fair Value	39,290	41,337
GNMA [Member] \| Residential Related Securities [Member]
Investment securities available for sale:
Amortized Cost	1,798,096	1,605,956
Gross Unrealized Gains	14,183	507
Gross Unrealized Losses	(750)	(16,460)
Fair Value	1,811,529	1,590,003
Investment securities held to maturity:
Amortized Cost	86,141	82,994
Gross Unrealized Gains	1,703	189
Gross Unrealized Losses	(24)	(309)
Fair Value	$ 87,820	$ 82,874

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

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