Securities Available for Sale and Held to Maturity (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Securities available for sale [Abstract]
Amortized cost	$ 2,352,013	$ 2,189,005
Gross unrealized gains	34,131	31,979
Gross unrealized (losses)	(11,577)	(26,830)
Estimated fair value	2,374,567	2,194,154
Securities held to maturity [Abstract]
Amortized cost	1,170,685	1,168,728
Gross unrealized gains	15,156	3,063
Gross unrealized (losses)	(2,995)	(20,958)
Estimated fair value	1,182,846	1,150,833
US Treasury Securities [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 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	100,971	100,159
Gross unrealized gains	2,631	0
Gross unrealized (losses)	0	(1,580)
Estimated fair value	103,602	98,579
Obligations of States and Political Subdivisions [Member]
Securities held to maturity [Abstract]
Amortized cost	63,505	65,987
Gross unrealized gains	3,398	2,806
Gross unrealized (losses)	0	(281)
Estimated fair value	66,903	68,512
Residential Mortgage Pass-Through Securities Guaranteed by GNMA [Member]
Securities held to maturity [Abstract]
Amortized cost	19,115	9,433
Gross unrealized gains	466	142
Gross unrealized (losses)	(16)	(72)
Estimated fair value	19,565	9,503
Residential Mortgage Pass-Through Securities Issued by FNMA and FHLMC [Member]
Securities held to maturity [Abstract]
Amortized cost	11,437	12,724
Gross unrealized gains	471	30
Gross unrealized (losses)	0	(162)
Estimated fair value	11,908	12,592
Other Residential Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Securities held to maturity [Abstract]
Amortized cost	834,176	837,393
Gross unrealized gains	6,440	0
Gross unrealized (losses)	(1,916)	(15,072)
Estimated fair value	838,700	822,321
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Securities held to maturity [Abstract]
Amortized cost	141,481	143,032
Gross unrealized gains	1,750	85
Gross unrealized (losses)	(1,063)	(3,791)
Estimated fair value	142,168	139,326
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	501
Gross unrealized gains	0	1
Gross unrealized (losses)	0	0
Estimated fair value	100	502
U.S. Government Agency Obligations Issued by U.S. Government Agencies [Member]
Securities available for sale [Abstract]
Amortized cost	79,788	129,653
Gross unrealized gains	694	1,125
Gross unrealized (losses)	(826)	(1,485)
Estimated fair value	79,656	129,293
U.S. Government Agency Obligations Issued by U.S. Government Sponsored Agencies [Member]
Securities available for sale [Abstract]
Amortized cost	32,725	40,681
Gross unrealized gains	170	19
Gross unrealized (losses)	(77)	(521)
Estimated fair value	32,818	40,179
Obligations of States and Political Subdivisions [Member]
Securities available for sale [Abstract]
Amortized cost	157,001	165,810
Gross unrealized gains	5,325	6,243
Gross unrealized (losses)	(68)	(315)
Estimated fair value	162,258	171,738
Residential Mortgage Pass-Through Securities Guaranteed by GNMA [Member]
Securities available for sale [Abstract]
Amortized cost	11,897	14,099
Gross unrealized gains	544	459
Gross unrealized (losses)	(14)	(84)
Estimated fair value	12,427	14,474
Residential Mortgage Pass-Through Securities Issued by FNMA and FHLMC [Member]
Securities available for sale [Abstract]
Amortized cost	199,599	239,880
Gross unrealized gains	4,842	3,147
Gross unrealized (losses)	0	(1,909)
Estimated fair value	204,441	241,118
Other Residential Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Securities available for sale [Abstract]
Amortized cost	1,655,733	1,300,375
Gross unrealized gains	16,664	12,459
Gross unrealized (losses)	(10,564)	(22,093)
Estimated fair value	1,661,833	1,290,741
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Securities available for sale [Abstract]
Amortized cost	184,394	235,317
Gross unrealized gains	4,968	7,278
Gross unrealized (losses)	(28)	(423)
Estimated fair value	189,334	242,172
Asset-Backed Securities and Structured Financial Products [Member]
Securities available for sale [Abstract]
Amortized cost	30,776	62,689
Gross unrealized gains	924	1,248
Gross unrealized (losses)	0	0
Estimated fair value	$ 31,700	$ 63,937