Securities Available for Sale and Held to Maturity (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Available for sale securities [Abstract]
Amortized cost	$ 2,349,631	$ 2,189,005
Gross unrealized gains	32,102	31,979
Gross unrealized (losses)	(17,838)	(26,830)
Estimated fair value	2,363,895	2,194,154
Held to maturity securities [Abstract]
Amortized cost	1,169,640	1,168,728
Gross unrealized gains	8,573	3,063
Gross unrealized (losses)	(8,000)	(20,958)
Estimated fair value	1,170,213	1,150,833
US Treasury Securities [Member]
Held to maturity securities [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]
Held to maturity securities [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]
Held to maturity securities [Abstract]
Amortized cost	100,767	100,159
Gross unrealized gains	1,858	0
Gross unrealized (losses)	(55)	(1,580)
Estimated fair value	102,570	98,579
Obligations of States and Political Subdivisions [Member]
Held to maturity securities [Abstract]
Amortized cost	64,538	65,987
Gross unrealized gains	3,487	2,806
Gross unrealized (losses)	0	(281)
Estimated fair value	68,025	68,512
Residential Mortgage Pass Through Securities Guaranteed by GNMA [Member]
Held to maturity securities [Abstract]
Amortized cost	13,368	9,433
Gross unrealized gains	304	142
Gross unrealized (losses)	(15)	(72)
Estimated fair value	13,657	9,503
Residential Mortgage Pass Through Securities Issued by FNMA and FHLMC [Member]
Held to maturity securities [Abstract]
Amortized cost	11,816	12,724
Gross unrealized gains	245	30
Gross unrealized (losses)	0	(162)
Estimated fair value	12,061	12,592
Other Residential Mortgage Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Held to maturity securities [Abstract]
Amortized cost	836,966	837,393
Gross unrealized gains	1,723	0
Gross unrealized (losses)	(6,106)	(15,072)
Estimated fair value	832,583	822,321
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Held to maturity securities [Abstract]
Amortized cost	142,185	143,032
Gross unrealized gains	956	85
Gross unrealized (losses)	(1,824)	(3,791)
Estimated fair value	141,317	139,326
Asset-Backed Securities and Structured Financial Products [Member]
Held to maturity securities [Abstract]
Amortized cost	0	0
Gross unrealized gains	0	0
Gross unrealized (losses)	0	0
Estimated fair value	0	0
US Treasury Securities [Member]
Available for sale securities [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]
Available for sale securities [Abstract]
Amortized cost	83,354	129,653
Gross unrealized gains	729	1,125
Gross unrealized (losses)	(1,072)	(1,485)
Estimated fair value	83,011	129,293
U.S. Government Agency Obligations Issued by U.S. Government Sponsored Agencies [Member]
Available for sale securities [Abstract]
Amortized cost	30,719	40,681
Gross unrealized gains	100	19
Gross unrealized (losses)	(40)	(521)
Estimated fair value	30,779	40,179
Obligations of States and Political Subdivisions [Member]
Available for sale securities [Abstract]
Amortized cost	159,304	165,810
Gross unrealized gains	6,200	6,243
Gross unrealized (losses)	(41)	(315)
Estimated fair value	165,463	171,738
Residential Mortgage Pass Through Securities Guaranteed by GNMA [Member]
Available for sale securities [Abstract]
Amortized cost	12,319	14,099
Gross unrealized gains	524	459
Gross unrealized (losses)	(15)	(84)
Estimated fair value	12,828	14,474
Residential Mortgage Pass Through Securities Issued by FNMA and FHLMC [Member]
Available for sale securities [Abstract]
Amortized cost	209,455	239,880
Gross unrealized gains	4,086	3,147
Gross unrealized (losses)	(121)	(1,909)
Estimated fair value	213,420	241,118
Other Residential Mortgage Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Available for sale securities [Abstract]
Amortized cost	1,606,565	1,300,375
Gross unrealized gains	13,065	12,459
Gross unrealized (losses)	(16,492)	(22,093)
Estimated fair value	1,603,138	1,290,741
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Available for sale securities [Abstract]
Amortized cost	215,284	235,317
Gross unrealized gains	6,414	7,278
Gross unrealized (losses)	(57)	(423)
Estimated fair value	221,641	242,172
Asset-Backed Securities and Structured Financial Products [Member]
Available for sale securities [Abstract]
Amortized cost	32,531	62,689
Gross unrealized gains	984	1,248
Gross unrealized (losses)	0	0
Estimated fair value	$ 33,515	$ 63,937