Securities Available for Sale and Held to Maturity (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Securities Available for Sale [Abstract]
Amortized Cost	$ 2,340,467	$ 2,352,013
Gross Unrealized Gains	46,637	34,131
Gross Unrealized (Losses)	(5,645)	(11,577)
Estimated Fair Value	2,381,459	2,374,567
Securities Held to Maturity [Abstract]
Amortized Cost	1,184,554	1,170,685
Gross Unrealized Gains	28,197	15,156
Gross Unrealized (Losses)	(246)	(2,995)
Estimated Fair Value	1,212,505	1,182,846
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	101,171	100,971
Gross Unrealized Gains	4,358	2,631
Gross Unrealized (Losses)	0	0
Estimated Fair Value	105,529	103,602
Obligations of States and Political Subdivisions [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	62,928	63,505
Gross Unrealized Gains	3,739	3,398
Gross Unrealized (Losses)	0	0
Estimated Fair Value	66,667	66,903
Residential Mortgage Pass Through Securities Guaranteed by GNMA [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	18,861	19,115
Gross Unrealized Gains	548	466
Gross Unrealized (Losses)	(6)	(16)
Estimated Fair Value	19,403	19,565
Residential Mortgage Pass Through Securities Issued by FNMA and FHLMC [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	11,341	11,437
Gross Unrealized Gains	578	471
Gross Unrealized (Losses)	0	0
Estimated Fair Value	11,919	11,908
Other Residential Mortgage Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	842,827	834,176
Gross Unrealized Gains	15,784	6,440
Gross Unrealized (Losses)	(151)	(1,916)
Estimated Fair Value	858,460	838,700
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Securities Held to Maturity [Abstract]
Amortized Cost	147,426	141,481
Gross Unrealized Gains	3,190	1,750
Gross Unrealized (Losses)	(89)	(1,063)
Estimated Fair Value	150,527	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
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	77,812	79,788
Gross Unrealized Gains	860	694
Gross Unrealized (Losses)	(557)	(826)
Estimated Fair Value	78,115	79,656
U.S. Government Agency Obligations Issued by U.S. Government Sponsored Agencies [Member]
Securities Available for Sale [Abstract]
Amortized Cost	32,737	32,725
Gross Unrealized Gains	339	170
Gross Unrealized (Losses)	0	(77)
Estimated Fair Value	33,076	32,818
Obligations of States and Political Subdivisions [Member]
Securities Available for Sale [Abstract]
Amortized Cost	154,591	157,001
Gross Unrealized Gains	5,595	5,325
Gross Unrealized (Losses)	(32)	(68)
Estimated Fair Value	160,154	162,258
Residential Mortgage Pass Through Securities Guaranteed by GNMA [Member]
Securities Available for Sale [Abstract]
Amortized Cost	11,403	11,897
Gross Unrealized Gains	613	544
Gross Unrealized (Losses)	(6)	(14)
Estimated Fair Value	12,010	12,427
Residential Mortgage Pass Through Securities Issued by FNMA and FHLMC [Member]
Securities Available for Sale [Abstract]
Amortized Cost	189,935	199,599
Gross Unrealized Gains	5,536	4,842
Gross Unrealized (Losses)	(1)	0
Estimated Fair Value	195,470	204,441
Other Residential Mortgage Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Securities Available for Sale [Abstract]
Amortized Cost	1,625,597	1,655,733
Gross Unrealized Gains	26,162	16,664
Gross Unrealized (Losses)	(5,049)	(10,564)
Estimated Fair Value	1,646,710	1,661,833
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Securities Available for Sale [Abstract]
Amortized Cost	219,149	184,394
Gross Unrealized Gains	6,677	4,968
Gross Unrealized (Losses)	0	(28)
Estimated Fair Value	225,826	189,334
Asset-Backed Securities and Structured Financial Products [Member]
Securities Available for Sale [Abstract]
Amortized Cost	29,243	30,776
Gross Unrealized Gains	855	924
Gross Unrealized (Losses)	0	0
Estimated Fair Value	$ 30,098	$ 31,700