Securities Available for Sale and Held to Maturity (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Available for sale securities [Abstract]
Amortized cost	$ 2,370,833	$ 2,189,005
Gross unrealized gains	31,882	31,979
Gross unrealized (losses)	(20,274)	(26,830)
Estimated fair value	2,382,441	2,194,154
Held to maturity securities [Abstract]
Amortized cost	1,155,569	1,168,728
Gross unrealized gains	6,818	3,063
Gross unrealized (losses)	(7,781)	(20,958)
Estimated fair value	1,154,606	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	100,361	100,159
Gross unrealized gains	292	0
Gross unrealized (losses)	(120)	(1,580)
Estimated fair value	100,533	98,579
Obligations of States and Political Subdivisions [Member]
Held to maturity securities [Abstract]
Amortized cost	65,757	65,987
Gross unrealized gains	3,032	2,806
Gross unrealized (losses)	0	(281)
Estimated fair value	68,789	68,512
Residential Mortgage Pass Through Securities Guaranteed by GNMA [Member]
Held to maturity securities [Abstract]
Amortized cost	12,177	9,433
Gross unrealized gains	201	142
Gross unrealized (losses)	(2)	(72)
Estimated fair value	12,376	9,503
Residential Mortgage Pass Through Securities Issued by FNMA and FHLMC [Member]
Held to maturity securities [Abstract]
Amortized cost	12,395	12,724
Gross unrealized gains	37	30
Gross unrealized (losses)	(10)	(162)
Estimated fair value	12,422	12,592
Other Residential Mortgage Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Held to maturity securities [Abstract]
Amortized cost	822,135	837,393
Gross unrealized gains	3,020	0
Gross unrealized (losses)	(5,160)	(15,072)
Estimated fair value	819,995	822,321
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Held to maturity securities [Abstract]
Amortized cost	142,744	143,032
Gross unrealized gains	236	85
Gross unrealized (losses)	(2,489)	(3,791)
Estimated fair value	140,491	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	124,095	129,653
Gross unrealized gains	813	1,125
Gross unrealized (losses)	(1,540)	(1,485)
Estimated fair value	123,368	129,293
U.S. Government Agency Obligations Issued by U.S. Government Sponsored Agencies [Member]
Available for sale securities [Abstract]
Amortized cost	40,694	40,681
Gross unrealized gains	40	19
Gross unrealized (losses)	(133)	(521)
Estimated fair value	40,601	40,179
Obligations of States and Political Subdivisions [Member]
Available for sale securities [Abstract]
Amortized cost	166,161	165,810
Gross unrealized gains	6,452	6,243
Gross unrealized (losses)	(176)	(315)
Estimated fair value	172,437	171,738
Residential Mortgage Pass Through Securities Guaranteed by GNMA [Member]
Available for sale securities [Abstract]
Amortized cost	13,771	14,099
Gross unrealized gains	541	459
Gross unrealized (losses)	(49)	(84)
Estimated fair value	14,263	14,474
Residential Mortgage Pass Through Securities Issued by FNMA and FHLMC [Member]
Available for sale securities [Abstract]
Amortized cost	229,915	239,880
Gross unrealized gains	3,511	3,147
Gross unrealized (losses)	(938)	(1,909)
Estimated fair value	232,488	241,118
Other Residential Mortgage Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Available for sale securities [Abstract]
Amortized cost	1,535,141	1,300,375
Gross unrealized gains	12,173	12,459
Gross unrealized (losses)	(17,246)	(22,093)
Estimated fair value	1,530,068	1,290,741
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Available for sale securities [Abstract]
Amortized cost	225,065	235,317
Gross unrealized gains	7,199	7,278
Gross unrealized (losses)	(192)	(423)
Estimated fair value	232,072	242,172
Asset-Backed Securities and Structured Financial Products [Member]
Available for sale securities [Abstract]
Amortized cost	35,891	62,689
Gross unrealized gains	1,153	1,248
Gross unrealized (losses)	0	0
Estimated fair value	$ 37,044	$ 63,937