Securities Available for Sale and Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Securities Available for Sale [Abstract]
Amortized Cost	$ 2,346,550	$ 2,352,013
Gross Unrealized Gains	41,700	34,131
Gross Unrealized (Losses)	(5,428)	(11,577)
Estimated Fair Value	2,382,822	2,374,567
Securities Held to Maturity [Abstract]
Amortized Cost	1,178,440	1,170,685
Gross Unrealized Gains	25,026	15,156
Gross Unrealized (Losses)	(398)	(2,995)
Estimated Fair Value	1,203,068	1,182,846
US Treasury Securities [Member]
Securities Available for Sale [Abstract]
Amortized Cost		100
Gross Unrealized Gains		0
Gross Unrealized (Losses)		0
Estimated Fair Value		100
U.S. Government Agency Obligations Issued by U.S. Government Agencies [Member]
Securities Available for Sale [Abstract]
Amortized Cost	70,993	79,788
Gross Unrealized Gains	716	694
Gross Unrealized (Losses)	(427)	(826)
Estimated Fair Value	71,282	79,656
U.S. Government Agency Obligations Issued by U.S. Government Sponsored Agencies [Member]
Securities Available for Sale [Abstract]
Amortized Cost	22,774	32,725
Gross Unrealized Gains	242	170
Gross Unrealized (Losses)	0	(77)
Estimated Fair Value	23,016	32,818
Obligations of States and Political Subdivisions [Member]
Securities Available for Sale [Abstract]
Amortized Cost	142,968	157,001
Gross Unrealized Gains	4,839	5,325
Gross Unrealized (Losses)	(13)	(68)
Estimated Fair Value	147,794	162,258
Residential Mortgage Pass-Through Securities Guaranteed by GNMA [Member]
Securities Available for Sale [Abstract]
Amortized Cost	26,269	11,897
Gross Unrealized Gains	486	544
Gross Unrealized (Losses)	(104)	(14)
Estimated Fair Value	26,651	12,427
Residential Mortgage Pass-Through Securities Issued by FNMA and FHLMC [Member]
Securities Available for Sale [Abstract]
Amortized Cost	172,710	199,599
Gross Unrealized Gains	4,722	4,842
Gross Unrealized (Losses)	(21)	0
Estimated Fair Value	177,411	204,441
Other Residential-Mortgage Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Securities Available for Sale [Abstract]
Amortized Cost	1,611,762	1,655,733
Gross Unrealized Gains	23,413	16,664
Gross Unrealized (Losses)	(4,773)	(10,564)
Estimated Fair Value	1,630,402	1,661,833
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Securities Available for Sale [Abstract]
Amortized Cost	272,779	184,394
Gross Unrealized Gains	6,920	4,968
Gross Unrealized (Losses)	(90)	(28)
Estimated Fair Value	279,609	189,334
Asset-Backed Securities and Structured Financial Products [Member]
Securities Available for Sale [Abstract]
Amortized Cost	26,295	30,776
Gross Unrealized Gains	362	924
Gross Unrealized (Losses)	0	0
Estimated Fair Value	26,657	31,700
US Treasury Securities [Member]
Securities Held to Maturity [Abstract]
Amortized Cost		0
Gross Unrealized Gains		0
Gross Unrealized (Losses)		0
Estimated Fair Value		0
U.S. Government Agency Obligations Issued by U.S. Government Agencies [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	0	0
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	0	0
Estimated Fair Value	0	0
U.S. Government Agency Obligations Issued by U.S. Government Sponsored Agencies [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	101,578	100,971
Gross Unrealized Gains	4,380	2,631
Gross Unrealized (Losses)	0	0
Estimated Fair Value	105,958	103,602
Obligations of States and Political Subdivisions [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	56,661	63,505
Gross Unrealized Gains	3,242	3,398
Gross Unrealized (Losses)	0	0
Estimated Fair Value	59,903	66,903
Residential Mortgage Pass-Through Securities Guaranteed by GNMA [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	17,783	19,115
Gross Unrealized Gains	435	466
Gross Unrealized (Losses)	(19)	(16)
Estimated Fair Value	18,199	19,565
Residential Mortgage Pass-Through Securities Issued by FNMA and FHLMC [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	10,669	11,437
Gross Unrealized Gains	457	471
Gross Unrealized (Losses)	0	0
Estimated Fair Value	11,126	11,908
Other Residential-Mortgage Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	808,763	834,176
Gross Unrealized Gains	12,196	6,440
Gross Unrealized (Losses)	(315)	(1,916)
Estimated Fair Value	820,644	838,700
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	182,986	141,481
Gross Unrealized Gains	4,316	1,750
Gross Unrealized (Losses)	(64)	(1,063)
Estimated Fair Value	187,238	142,168
Asset-Backed Securities and Structured Financial Products [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	0	0
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	0	0
Estimated Fair Value	$ 0	$ 0