Securities Available for Sale and Held to Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Securities Available for Sale [Abstract]
Amortized Cost	$ 2,427,983	$ 2,352,013
Gross Unrealized Gains	31,231	34,131
Gross Unrealized (Losses)	(12,831)	(11,577)
Estimated Fair Value	2,446,383	2,374,567
Securities Held to Maturity [Abstract]
Amortized Cost	1,190,161	1,170,685
Gross Unrealized Gains	14,353	15,156
Gross Unrealized (Losses)	(4,297)	(2,995)
Estimated Fair Value	1,200,217	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	74,258	79,788
Gross Unrealized Gains	808	694
Gross Unrealized (Losses)	(657)	(826)
Estimated Fair Value	74,409	79,656
U.S. Government Agency Obligations Issued by U.S. Government Sponsored Agencies [Member]
Securities Available for Sale [Abstract]
Amortized Cost	32,748	32,725
Gross Unrealized Gains	261	170
Gross Unrealized (Losses)	0	(77)
Estimated Fair Value	33,009	32,818
Obligations of States and Political Subdivisions [Member]
Securities Available for Sale [Abstract]
Amortized Cost	146,711	157,001
Gross Unrealized Gains	4,668	5,325
Gross Unrealized (Losses)	(57)	(68)
Estimated Fair Value	151,322	162,258
Residential Mortgage Pass-Through Securities Guaranteed by GNMA [Member]
Securities Available for Sale [Abstract]
Amortized Cost	20,349	11,897
Gross Unrealized Gains	508	544
Gross Unrealized (Losses)	(206)	(14)
Estimated Fair Value	20,651	12,427
Residential Mortgage Pass-Through Securities Issued by FNMA and FHLMC [Member]
Securities Available for Sale [Abstract]
Amortized Cost	181,658	199,599
Gross Unrealized Gains	4,150	4,842
Gross Unrealized (Losses)	(157)	0
Estimated Fair Value	185,651	204,441
Other Residential-Mortgage Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Securities Available for Sale [Abstract]
Amortized Cost	1,658,085	1,655,733
Gross Unrealized Gains	14,991	16,664
Gross Unrealized (Losses)	(10,600)	(10,564)
Estimated Fair Value	1,662,476	1,661,833
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Securities Available for Sale [Abstract]
Amortized Cost	286,496	184,394
Gross Unrealized Gains	5,056	4,968
Gross Unrealized (Losses)	(1,154)	(28)
Estimated Fair Value	290,398	189,334
Asset-Backed Securities and Structured Financial Products [Member]
Securities Available for Sale [Abstract]
Amortized Cost	27,678	30,776
Gross Unrealized Gains	789	924
Gross Unrealized (Losses)	0	0
Estimated Fair Value	28,467	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	101,374	100,971
Gross Unrealized Gains	3,278	2,631
Gross Unrealized (Losses)	0	0
Estimated Fair Value	104,652	103,602
Obligations of States and Political Subdivisions [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	56,978	63,505
Gross Unrealized Gains	2,972	3,398
Gross Unrealized (Losses)	0	0
Estimated Fair Value	59,950	66,903
Residential Mortgage Pass-Through Securities Guaranteed by GNMA [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	18,265	19,115
Gross Unrealized Gains	335	466
Gross Unrealized (Losses)	(82)	(16)
Estimated Fair Value	18,518	19,565
Residential Mortgage Pass-Through Securities Issued by FNMA and FHLMC [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	10,965	11,437
Gross Unrealized Gains	320	471
Gross Unrealized (Losses)	0	0
Estimated Fair Value	11,285	11,908
Other Residential-Mortgage Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	838,989	834,176
Gross Unrealized Gains	5,496	6,440
Gross Unrealized (Losses)	(3,390)	(1,916)
Estimated Fair Value	841,095	838,700
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	163,590	141,481
Gross Unrealized Gains	1,952	1,750
Gross Unrealized (Losses)	(825)	(1,063)
Estimated Fair Value	164,717	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