Investment Securities, AFS and HTM Securities Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Investment securities available for sale:
Amortized Cost	$ 4,865,970	$ 4,984,867
Gross Unrealized Gains	52,349	35,684
Gross Unrealized Losses	(12,478)	(53,137)
Fair Value	4,905,841	4,967,414
Investment securities held to maturity:
Amortized Cost	1,176,821	1,168,230
Gross Unrealized Gains	27,855	17,505
Gross Unrealized Losses	(331)	(1,293)
Fair Value	1,204,345	1,184,442
US Treasury Securities [Member]
Investment securities available for sale:
Amortized Cost	1,000	999
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(2)
Fair Value	1,000	997
Obligations of state and political subdivisions (municipal securities) [Member]
Investment securities held to maturity:
Amortized Cost	1,054,757	1,043,767
Gross Unrealized Gains	25,972	16,803
Gross Unrealized Losses	(146)	(339)
Fair Value	1,080,583	1,060,231
Private Label [Member]
Investment securities available for sale:
Amortized Cost	1,535	1,722
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(23)	(14)
Fair Value	1,512	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	2,053,540	1,982,477
Gross Unrealized Gains	5,855	1,334
Gross Unrealized Losses	(11,530)	(28,501)
Fair Value	2,047,865	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	87	51
Gross Unrealized Losses	0	0
Fair Value	4,805	4,769
FNMA and FHLMC [Member] \| Residential Related Securities [Member]
Investment securities available for sale:
Amortized Cost	1,268,929	1,388,995
Gross Unrealized Gains	35,321	33,791
Gross Unrealized Losses	(846)	(8,160)
Fair Value	1,303,404	1,414,626
Investment securities held to maturity:
Amortized Cost	40,404	41,469
Gross Unrealized Gains	797	513
Gross Unrealized Losses	(180)	(645)
Fair Value	41,021	41,337
GNMA [Member] \| Residential Related Securities [Member]
Investment securities available for sale:
Amortized Cost	1,536,248	1,605,956
Gross Unrealized Gains	11,086	507
Gross Unrealized Losses	(79)	(16,460)
Fair Value	1,547,255	1,590,003
Investment securities held to maturity:
Amortized Cost	81,660	82,994
Gross Unrealized Gains	1,086	189
Gross Unrealized Losses	(5)	(309)
Fair Value	$ 82,741	$ 82,874

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

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