Securities Available for Sale and Held to Maturity (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Available for sale securities [Abstract]
Amortized cost	$ 2,395,263,000	$ 2,143,074,000
Gross unrealized gains	73,918,000	50,079,000
Gross unrealized (losses)	(188,000)	(15,904,000)
Estimated fair value	2,468,993,000	2,177,249,000
Held to maturity securities [Abstract]
Amortized cost	57,705,000	140,847,000
Gross unrealized gains	4,813,000	4,308,000
Gross unrealized (losses)	(3,000)	(12,000)
Estimated fair value	62,515,000	145,143,000
Fair value [Abstract]
Estimated fair value, less than 12 months	51,327,000	718,579,000
Estimated fair value, 12 months or more	202,000	307,000
Total estimated fair value	51,529,000	718,886,000
Gross unrealized losses [Abstract]
Gross unrealized (losses), less than 12 months	(189,000)	(15,911,000)
Gross unrealized (losses), 12 months or more	(2,000)	(5,000)
Total gross unrealized (losses)	(191,000)	(15,916,000)
Gains (losses) on investments [Abstract]
Pledged to collateralize public deposits and securities sold under repurchase agreements and for other purposes as permitted by law	1,800,000,000	1,700,000,000
Pledged securities providing additional contingency funding	6,400,000
Available for Sale, estimated fair value [Abstract]
Securities available for sale (at fair value)	2,468,993,000	2,177,249,000
Held to maturity, amortized cost [Abstract]
Securities held to maturity	57,705,000	140,847,000
Available-for-sale Securities [Member]
Gains (losses) on investments [Abstract]
Proceeds from calls and sales of securities	24,471,000	65,074,000	188,460,000
Gross realized gains - Available for Sale	57,000	2,216,000	5,379,000
Gross realized (losses) - Available for Sale	(11,000)	0	(11,000)
Available for Sale, amortized cost [Abstract]
Due in one year or less	9,506,000
Due after one year through five years	59,092,000
Due after five years through ten years	176,050,000
Due after ten years	10,796,000
Total amortized cost, before mortgage-backed-securities	255,444,000
Mortgage-backed securities	2,139,819,000
Total amortized cost	2,395,263,000
Available for Sale, estimated fair value [Abstract]
Due in one year or less	9,568,000
Due after one year through five years	61,549,000
Due after five years through ten years	185,097,000
Due after ten years	11,418,000
Total fair value, before mortgage-backed-securities	267,632,000
Mortgage-backed securities	2,201,361,000
Securities available for sale (at fair value)	2,468,993,000
Held-to-maturity Securities [Member]
Gains (losses) on investments [Abstract]
Proceeds from calls of securities	3,645,000	11,305,000	9,303,000
Gross realized gains - Held to Maturity	34,000	113,000	99,000
Held to maturity, amortized cost [Abstract]
Due in one year or less	2,922,000
Due after one year through five years	18,377,000
Due after five years through ten years	19,824,000
Due after ten years	1,496,000
Total amortized cost, before mortgage-backed-securities	42,619,000
Mortgage-backed securities	15,086,000
Securities held to maturity	57,705,000
Held to maturity, estimated fair value [Abstract]
Due in one year or less	2,942,000
Due after one year through five years	19,922,000
Due after five years through ten years	22,298,000
Due after ten years	1,586,000
Total fair value, before mortgage-backed-securities	46,748,000
Mortgage-backed securities	15,767,000
Securities held to maturity (at fair value)	62,515,000
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	0	0
Gross unrealized gains	0	0
Gross unrealized (losses)	0	0
Estimated fair value	0	0
Fair value [Abstract]
Estimated fair value, less than 12 months		86,917,000
Estimated fair value, 12 months or more		0
Total estimated fair value		86,917,000
Gross unrealized losses [Abstract]
Gross unrealized (losses), less than 12 months		(2,184,000)
Gross unrealized (losses), 12 months or more		0
Total gross unrealized (losses)		(2,184,000)
Obligations of States and Political Subdivisions [Member]
Held to maturity securities [Abstract]
Amortized cost	42,619,000	53,246,000
Gross unrealized gains	4,131,000	2,628,000
Gross unrealized (losses)	(2,000)	(10,000)
Estimated fair value	46,748,000	55,864,000
Fair value [Abstract]
Estimated fair value, less than 12 months	3,368,000	65,523,000
Estimated fair value, 12 months or more	202,000	307,000
Total estimated fair value	3,570,000	65,830,000
Gross unrealized losses [Abstract]
Gross unrealized (losses), less than 12 months	(12,000)	(2,045,000)
Gross unrealized (losses), 12 months or more	(2,000)	(5,000)
Total gross unrealized (losses)	(14,000)	(2,050,000)
Residential Mortgage Pass-Through Securities Guaranteed by GNMA [Member]
Held to maturity securities [Abstract]
Amortized cost	4,538,000	6,058,000
Gross unrealized gains	336,000	171,000
Gross unrealized (losses)	0	0
Estimated fair value	4,874,000	6,229,000
Residential Mortgage Pass-Through Securities Issued by FNMA and FHLMC [Member]
Held to maturity securities [Abstract]
Amortized cost	588,000	763,000
Gross unrealized gains	28,000	6,000
Gross unrealized (losses)	0	0
Estimated fair value	616,000	769,000
Fair value [Abstract]
Estimated fair value, less than 12 months		312,787,000
Estimated fair value, 12 months or more		0
Total estimated fair value		312,787,000
Gross unrealized losses [Abstract]
Gross unrealized (losses), less than 12 months		(6,846,000)
Gross unrealized (losses), 12 months or more		0
Total gross unrealized (losses)		(6,846,000)
Other Residential Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Held to maturity securities [Abstract]
Amortized cost	7,749,000	78,526,000
Gross unrealized gains	133,000	1,503,000
Gross unrealized (losses)	(1,000)	0
Estimated fair value	7,881,000	80,029,000
Fair value [Abstract]
Estimated fair value, less than 12 months	1,069,000	232,279,000
Estimated fair value, 12 months or more	0	0
Total estimated fair value	1,069,000	232,279,000
Gross unrealized losses [Abstract]
Gross unrealized (losses), less than 12 months	(1,000)	(4,311,000)
Gross unrealized (losses), 12 months or more	0	0
Total gross unrealized (losses)	(1,000)	(4,311,000)
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Held to maturity securities [Abstract]
Amortized cost	2,211,000	2,254,000
Gross unrealized gains	185,000	0
Gross unrealized (losses)	0	(2,000)
Estimated fair value	2,396,000	2,252,000
Fair value [Abstract]
Estimated fair value, less than 12 months	46,890,000	21,073,000
Estimated fair value, 12 months or more	0	0
Total estimated fair value	46,890,000	21,073,000
Gross unrealized losses [Abstract]
Gross unrealized (losses), less than 12 months	(176,000)	(525,000)
Gross unrealized (losses), 12 months or more	0	0
Total gross unrealized (losses)	(176,000)	(525,000)
U.S. Government Agency Obligations Issued by U.S. Government Agencies [Member]
Available for sale securities [Abstract]
Amortized cost	3,000	12,000
Gross unrealized gains	0	0
Gross unrealized (losses)	0	0
Estimated fair value	3,000	12,000
U.S. Government Agency Obligations Issued by U.S. Government Sponsored Agencies [Member]
Available for sale securities [Abstract]
Amortized cost	64,573,000	124,093,000
Gross unrealized gains	229,000	114,000
Gross unrealized (losses)	0	(2,184,000)
Estimated fair value	64,802,000	122,023,000
Obligations of States and Political Subdivisions [Member]
Available for sale securities [Abstract]
Amortized cost	190,868,000	159,418,000
Gross unrealized gains	11,971,000	2,259,000
Gross unrealized (losses)	(12,000)	(2,040,000)
Estimated fair value	202,827,000	159,637,000
Residential Mortgage Pass-Through Securities Guaranteed by GNMA [Member]
Available for sale securities [Abstract]
Amortized cost	11,500,000	11,719,000
Gross unrealized gains	945,000	723,000
Gross unrealized (losses)	0	0
Estimated fair value	12,445,000	12,442,000
Residential Mortgage Pass-Through Securities Issued by FNMA and FHLMC [Member]
Available for sale securities [Abstract]
Amortized cost	340,839,000	432,162,000
Gross unrealized gains	7,093,000	1,188,000
Gross unrealized (losses)	0	(6,846,000)
Estimated fair value	347,932,000	426,504,000
Other Residential Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Available for sale securities [Abstract]
Amortized cost	1,570,782,000	1,361,339,000
Gross unrealized gains	44,183,000	43,788,000
Gross unrealized (losses)	0	(4,311,000)
Estimated fair value	1,614,965,000	1,400,816,000
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Available for sale securities [Abstract]
Amortized cost	216,698,000	54,331,000
Gross unrealized gains	9,497,000	2,007,000
Gross unrealized (losses)	(176,000)	(523,000)
Estimated fair value	$ 226,019,000	$ 55,815,000