Loans and Allowance for Credit Losses Part 1 (Details) (USD $)																				3 Months Ended
	Mar. 31, 2013		Dec. 31, 2012		Mar. 31, 2012		Mar. 31, 2013 Commercial [Member]	Dec. 31, 2012 Commercial [Member]	Mar. 31, 2012 Commercial [Member]	Mar. 31, 2013 Commercial [Member] Energy [Member]	Mar. 31, 2013 Commercial [Member] Services [Member]	Mar. 31, 2013 Commercial Real Estate [Member]	Dec. 31, 2012 Commercial Real Estate [Member]	Mar. 31, 2012 Commercial Real Estate [Member]	Mar. 31, 2013 Residential Mortgage [Member]	Dec. 31, 2012 Residential Mortgage [Member]	Mar. 31, 2012 Residential Mortgage [Member]	Mar. 31, 2013 Residential Mortgage [Member] Permanent mortgage [Member]	Mar. 31, 2013 Residential Mortgage [Member] Permanent mortgages guaranteed by U.S. government agencies [Member]	Mar. 31, 2013 Residential Mortgage [Member] Home equity [Member]	Mar. 31, 2013 Consumer [Member]	Dec. 31, 2012 Consumer [Member]	Mar. 31, 2012 Consumer [Member]
Loan receivables disclosure [Abstract]
Loans receivables with fixed rate of interest	$ 6,879,703,000		$ 6,926,021,000		$ 6,224,469,000		$ 4,104,169,000	$ 4,158,548,000	$ 3,491,029,000			$ 889,397,000	$ 845,023,000	$ 857,059,000	$ 1,732,058,000	$ 1,747,038,000	$ 1,662,585,000				$ 154,079,000	$ 175,412,000	$ 213,796,000
Loans receivables with variable rate of interest	5,081,228,000		5,251,025,000		5,169,616,000		3,294,275,000	3,458,897,000	3,390,806,000			1,330,588,000	1,323,350,000	1,308,765,000	234,966,000	251,394,000	278,879,000				221,399,000	217,384,000	191,166,000
Loan receivable non-accrual	132,633,000		134,410,000		183,359,000		19,861,000	24,467,000	61,750,000			65,175,000	60,626,000	86,475,000	45,426,000	46,608,000	27,462,000				2,171,000	2,709,000	7,672,000
Total	12,093,564,000		12,311,456,000		11,577,444,000		7,418,305,000	7,641,912,000	6,943,585,000	2,300,000,000	2,100,000,000	2,285,160,000	2,228,999,000	2,252,299,000	2,012,450,000	2,045,040,000	1,968,926,000			758,000,000	377,649,000	395,505,000	412,634,000
Accruing loans past due (90 days)	4,229,000	[1]	3,925,000	[1]	6,140,000	[1]
Loans receivable, other information [Abstract]
Amount of total loan portfolio to businesses and individuals in Oklahoma	5,100,000,000
Percentage of total loan portfolio to businesses and individuals in Oklahoma (in hundredths)	42.00%
Amount of total loan portfolio to businesses and individuals in Texas	3,900,000,000
Percentage of total loan portfolio to businesses and individuals in Texas (in hundredths)	32.00%
Amount of loan portfolio to businesses in Oklahoma							2,900,000,000
Percentage of loan portfolio to businesses in Oklahoma (in hundredths)							38.00%
Amount of loan portfolio to businesses in Texas							2,700,000,000
Percentage of loan portfolio to businesses in Texas (in hundredths)							37.00%
Percentage of energy loans in loan portfolio										19.00%
Amount of loans to energy producers										2,100,000,000
Percentage of committed energy production loans secured by oil										58.00%
Percentage of committed energy production loans secured by natural gas										42.00%
Amount of loans with individual balances less than $10 million											1,200,000,000
Maximum loan amount for certain individual loans in category											10,000,000
Percentage of loan portfolio secured by property in Texas (in hundredths)												35.00%
Percentage of loan portfolio secured by property in Oklahoma (in hundredths)												25.00%
Minimum FICO required for jumbo loan approval																		720
Maximum debt-to-income ratio on jumbo loans (in hundredths)																		38.00%
Minimum loan-to-value ratio on jumbo loans (in hundredths)																		60.00%
Maximum loan-to-value ratio on jumbo loans (in hundredths)																		100.00%
Minimum period for fixed rate on variable rate jumbo loans (in years)																		3 years
Maximum period for fixed rate on variable rate jumbo loans (in years)																		10 years
Amount of loans guaranteed by US government and previously sold into GNMA mortgage pools																			162,000,000
Percentage of home equity portfolio comprised of junior lien loans																				31.00%
Percentage of home equity portfolio comprised of first lien loans																				69.00%
Percentage of junior lien home equity loans that are amortizing term loans																				78.00%
Percentage of junior lien home equity loans that are revolving lines of credit																				22.00%
Home equity loans, description																				Home equity loans generally require a minimum FICO score of 700 and a maximum DTI of 40%.
Home equity loans, maximum																				400,000
Home equity loans, revolving period																				5 years
Home equity loans, amortization period following revolving period																				15 years
Home equity loans, discretionary additional revolving period																				5 years
Credit Commitments [Abstract]
Outstanding commitments to extend credit	6,900,000,000
Outstanding standby letters of credit	491,000,000
Outstanding commercial letters of credit	$ 8,000,000

[1] Excludes residential mortgage loans guaranteed by agencies of the U.S. government